Great post, thank you!
SAD: When I did a very brief lit search, the research showed much larger effects of vitamin D supplementation than light exposure therapy. Of course, they weren't using enough dakka on the light, so both should be used. But two of my close friends with severe SAD were dramatically improved when I got them to supplement D regularly. It's handy that you don't need to take it regularly, just in large doses occasionally (probably don't do more than 50k IU at a time for safety). Sorry I didn't keep the references where I can find them!
Again, doing both is probably a good idea, but most people seem to be vit. D deficient, as you'd expect from a light-exposure-synthesized vitamin, with all of this modern unnatural clothes-wearing and indoors-dwelling.
Back to light: as the standard male night owl (particularly on a WFH flexible schedule): Am I understanding you correctly that if I wanted to go to bed earlier (not sure I do but I probably should), I'd wake up earlier and blast my eyeballs with light right away, then avoid bright light 3-4 hours before bed? Anything else?
Thanks a lot, Seth!
I agree with you on the importance of Vitamin D—it has helped me enormously. Most studies on Vitamin D focus on achieving levels similar to those experienced during summertime. However, few of the bright light studies come anywhere close to replicating the typical lux levels experienced in summer. From my anecdotal experience, I’ve found that spending several hours under 10,000 lux+ artificial lights can totally transform my mood.
You are correct in your understanding of things to try as a night owl. One other thing you might consider is supplementing with 0.5mg of melatonin around 6pm [1]. Exogenous melatonin also has a phase response curve for affecting your circadian rhythm. It’s important to stick to a low dose; otherwise, you’re likely to feel drowsy in the evening.
Any reason for the timing window being 4 hours before instead of 30 min to 1 hour? Most of the stuff I've heard is around half an hour to an hour before bed, I'm currently doing this with 0.3ish mg (I divide a 1 mg tablet in 3) of melatonin.
Melatonin (particularly at higher doses) has a fast-acting sedative effect, so if you want to use it to help you fall asleep, then taking it 30 minutes before bed makes sense.
However, if you want to use it to advance your circadian rhythm (i.e., be inclined to be more of a morning person), then it has the greatest effect when taken 9–11 hours before your sleep midpoint (so very roughly at 6 p.m.). Below is a graph of the effect it has on the circadian rhythm.
^https://www.timeshifter.com/jet-lag/melatonin-for-jet-lag-type-dose-timing
I remember reading about SAD treatment by lumens in Inadequate Equilibria, though I did not finish the book.
This post paints a partially inaccurate picture. IMHO the following is more accurate.
Unless otherwise indicated, the following information comes from Andrew Huberman. Most comes from Huberman Lab Podcast #68. Huberman opines on a great many health topics. I want to stress that I don't consider Huberman a reliable authority in general, but I do consider him reliable on the circadian rhythm and on motivation and drive. (His research specialization for many years was the former and he for many years has successfully used various interventions to improve his own motivation and drive, which is very high.)
Bright light (especially bluish light) makes a person more alert. (Sufficiently acute exposure to cold, e.g., a plunge into 45-degree water, is an even stronger cause of alertness. Caffeine is another popular intervention for causing alertness.) After many hours of being alert and pursuing goals, a person will get tired, and this tiredness tends to help the person go to sleep. However, the SCN operates independently of exposure to bright light (and cold and caffeine) and independently of how many hours in a row the person has already been alert. A good illustration of that is what happens when a person pulls an all-nighter: at about 4:30 it becomes easier for most people pulling an all-nighter to stay awake even if the person is not being exposed to bright light and even if the person has already been awake for a very long time. Without any light as a stimulus, at around 04:30 the SCN decides to get the brain and the body ready for wakefulness and activity. So, let us inquire how the SCN would stay in sync with the sun in the ancestral environment before the availability of artificial lighting. How does the SCN know that dawn is coming soon?
The answer is that it is complicated, like most things in biology, but I think most neuroscientists agree that the stimulus that most potently entrains the SCN (i.e., that is most effective at ensuring the the SCN is in sync with the sun) is yellow-blue (YB) contrasts. Specifically, the SCN knows it is 4:30 and consequently time to start making the body alert because of the person's exposure to these "YB contrasts" on previous days. Exposure in the evening has an effect, but the strongest effect is exposure circa dawn.
When the sun is at a high angular elevation, it is white and the surrounding sky is dark blue (assuming a cloudless sky). When the sun is slightly above or below the horizon, the part of the sky near the sun is yellow or even orange or pink and with further (angular) distance from the sun, the sky gets steadily bluer. (Note that even 30 minutes before sunrise, the sky is already much brighter than your cell phone's screen or most indoor environments: there is an optical illusion whereby people underestimate the brightness of a light source when the source is spread over a large (angular) area and overestimate the brightness of "point sources" like light bulbs.)
The sensing of these YB contrasts is done by a system distinct from the usual visual system (i.e., the system that gives visual information that is immediately salient to the conscious mind) and in particular there are fewer "sensing pixels" and they are spread further apart than the "pixels" in the usual visual system. The final page of this next 5-page paper has a nice image of the author's estimate of a bunch of "sensing pixels" depicted as dotted circles laid over a photo of a typical sunrise:
https://pmc.ncbi.nlm.nih.gov/articles/PMC8407369/pdf/nihms-1719642.pdf
A light bulb named Tuo is recommended at least half-heartedly by Huberman for controlling the circadian rhythm. Huberman says IIRC it works by alternating between yellow light and blue light many times a second. Huberman explained IIRC that both "spatial" YB contrasts and "temporal" YB contrasts serve a signal that "dawn or dusk is happening". I definitely recall Huberman saying that outdoor light is preferred to this Tuo light bulb, and I understood him to mean that it is more likely to work because no one understands SCN entrainment well enough right now to design an artificial light source guaranteed to work.
The web site for this light bulb says,
Most light therapy products on the market today are based on blue light. This is 15-year-old science that has since been proven incorrect. New science based on laboratory research conducted by the University of Washington, one of the world's top vision and neuroscience centers, shows that blue light has little to no effect in shifting your circadian rhythm. High brightness levels of blue light can have some effect, but this falls short when compared to the power of TUO.
High lux products can work, but they require 10,000 lux of light at a distance of under 2 feet for the duration of treatment. This light level at this distance is uncomfortable for most people. Treatment also needs to happen first thing in the morning to be most effective. Who has time to sit within 2 feet of a bulb for up to a half hour when first waking up? High lux products offer dim settings to make use more comfortable and typically downplay the distance requirement. Unfortunately, at less than 10,000 lux and 2 feet of distance, high lux products have little to no impact.
Huberman recommends getting as much bright light early in the day as possible -- "preferably during the first hour after waking, but definitely during the first 3 hours". This encourages the body to produce cortisol and dopamine, which at this time of day is very good for you (and helps you be productive). But this bright light won't have much effect on keeping your circadian clock entrained with the schedule you want it be entrained with unless the light contains YB contrasts; i.e., getting bright light right after waking is good and getting sufficiently-bright light containing YB contrasts right after waking is also good, but they are good for different reasons (though the first kind of good contributes to a small extent to the second kind of good through the mechanism I described in my second paragraph).
Huberman is insistent that it is not enough to expose yourself to light after your normal wake-up time: you also have to avoid light when you are normally asleep. Suppose your normal wake-up time is 06:00. To maintain a strong circadian rhythm and to get to sleep at a regular time each night (which is good for you and which most people should strive to do) it is essential to avoid exposure to light during the 6 hours between 23:00 and 05:00. Whereas dim light has little positive effect after wake-up time, even quite dim light or light of brief duration between 23:00 and 05:00 tends to have pronounced negative effects.
Light during these 6 hours not only confuses the circadian clock (which is bad and makes it hard to get to sleep at a healthy hour) but it also decreases the amount of motivation and drive available the next morning (by sending signals to a brain region called the habenula). I personally have noticed a strong increase in my level of motivation and drive on most mornings after I instituted the habits described in this comment. (And I more reliably start my sleep at what I consider a healthy hour, but that was less critical in my case because insomnia was never a huge problem of mine.)
Huberman says that getting outside at dawn works to keep the SCN in sync with the sun even on completely overcast days, but it requires longer duration of exposure: 20 minutes instead of 5 minutes IIRC. He says that there are YB contrasts in the overcast sky when the sun is near the horizon that are absent when the angle of the sun is high.
To this point in this comment I merely repeated information I learned from Huberman (and maybe a bit from Wikipedia or such -- it is hard to remember) although I hasten to add that this information certainly jibes with my own experience of going outside at dawn almost every day starting about 2 years ago. Allow me to add one thing of my own invention, namely, what to call this 6-hour interval every night when it is a bad idea to let your eyes be exposed to light: I humbly suggest "curfew". The original meaning of "curfew" was a time every night during which it was illegal in medieval London to have a fire going even in your own fireplace in your own home. (I.e., it was a measure to prevent urban fires.)
Thanks for the detailed reply.
My understanding is that there are still significant unknowns on the exact mechanisms of entrainment, and I don’t dispute that yellow-blue (YB) contrasts play a role. I considered mentioning it in this post, but my understanding is that it is more of a secondary point compared to the significance of the timing of bright, blue light exposure. Curious to see any evidence for your/Huberman’s assertion that early morning light exposure in the absence of YB contrasts has little effect on entrainment. This seems to contradict most of the literature I’ve seen.
The balance of my post more closely reflects this 2021 summary of the state of the art by Russell Foster (who was crucial in the discovery of the role ipRGCs). I’m inclined to trust his overview of the literature over Huberman, who has spread himself quite thin in the past. Having said that am wary that this summary is from 2021 and am less familiar with research from the last couple of years….
If there is anything specific you think is factually inaccurate in the essay, I would be more than happy to discuss.
I know you just said that you don't completely trust Huberman, but just today, Huberman published a 30-minute video titled "Master your sleep and be more alert when awake". I listened to it (twice) to refresh my memory and to see if his advice changed.
He mentions yellow-blue (YB) contrasts once (at https://www.youtube.com/watch?v=lIo9FcrljDk&t=502s) and at least thrice he mentions the desirability of exposure to outdoor light when the sun is at a low angle (close to the horizon). As anyone can see by looking around at dawn and again at mid-day, at dawn some parts of the sky will be yellowish (particularly, the parts of the sky near the sun) or even orange whereas other parts will range from pale blue to something like turquoise to deep blue whereas at mid-day the sun is white, the part of the sky near the sun is blue and the blue parts of the sky are essentially all the same shade or hue of blue.
He also says that outdoor light (directly from the sun or indirectly via atmospheric scattering) is the best kind of light for maintaining a healthy circadian rhythm, but that if getting outdoors early enough that the sun is still low in the sky is impractical, artificial light can be effective, particularly blue-heavy artificial light.
I've been help greatly over the last 2 years by a protocol in which I get outdoor light almost every morning when the YB contrasts are at its most extreme, namely between about 20 min before sunrise and about 10 min after sunrise on clear days and a little later on cloudy days. (The other element of my protocol that I know to be essential is strictly limit my exposure to light between 23:00 and 05:00.) I was motivated to comment on your post because it did not contain enough information to help someone sufficiently similar to me (the me of 2 years ago) to achieve the very welcome results I achieved: I'm pretty sure that even very bright artificial light from ordinary LED lights that most of us have in our home (even very many of them shining all at once) would not have helped me nearly as much.
Huberman is not so insistent on getting outside during this 30-minute interval of maximum YB contrast as my protocol is. In fact in today's video he says that he himself often gets outside only after the sun has been out for an hour or 2 and is consequently no longer particularly near the horizon.
Health-conscious people apply a (software-based) filter to their screens in the evening to reduce blue light emitted from the screen. On iOS this is called Night Shift. If your rendition of the effects of light on the circadian rhythm (CR) is complete, then they're doing everything they can do, but if YB contrasts have important effects on the CR, it might be useful in addition to eliminate YB contrasts on our digital devices (which Night Shift and its analogs on the other platforms do not eliminate). This can be done by turning everything shades of gray. (On iOS for example, this can be achieved in Settings > Accessibility > Display & Text Size > Color Filters > Grayscale and can be combined with or "overlaid on" Night Shift.) I and others do this (turn of a filter that makes everything "grayscale") routinely to make it more likely that we will get sleepy sufficiently early in the evening. Additional people report that they like to keep their screens grayscale, but do not cite the CR as the reason for their doing so.
Is a computer screen bright enough such that YB contrasts on the screen can activate the machinery in the retina that is activated by a sunrise? I'm not sure, but I choose to eliminate YB contrasts on my screens just in case it is.
Finally let me quote what I consider the main takeaway from the video Huberman posted today, which I expect we both agree with:
Get up each morning, try to get outside. I know that can be challenging for people, but anywhere from 2 to 10 min of sun exposure will work well for most people. If you can't do it every day or if you sleep through this period of early-day low-solar angle, don't worry about it. The systems in the body -- these hormone systems and neurotransmitter systems -- that make you awake at certain periods of the day and sleepy at other times are operating by averaging when you view the brightest light.
You may have heard that you 'shouldn't use screens late in the evening' and maybe even that 'it's good for you to get exposure to sunshine as soon as possible after waking'. For the majority of people, these are generally beneficial heuristics. They are also the extent of most people's knowledge about how light affects their wellbeing.
The multiple mechanisms through which light affects our physiology make it hard to provide generalisable guidance. Among other things, the time of day, your genetics, your age, your mood and the brightness, frequency and duration of exposure to light all interrelate in determining how it affects us.
This document will explain some of the basic mechanisms through which light affects our physiology, with the goal of providing a framework to enable you to make informed decisions around your light exposure. After reading this, at any time on any given day, you should have a sense as to what type of light exposure you need right now. These decisions should lead to noticeable improvements in mood and productivity, whilst also improving sleep and reducing the risk of various long-term diseases.
Addressing SAD
Although SAD (Seasonal Affective Disorder) is a common framing used when describing the effect of light on health, I am going to largely avoid using the term here. Let me explain why...
Officially, SAD is a form of Major Depressive Disorder that comes and goes with seasonal patterns. Typically, this is characterised by depressive symptoms that occur in autumn and winter and resolve in spring and summer. [Confusingly, technically people who find themselves experiencing depression only in Summer also fall under the diagnosis of SAD].
One reason SAD is a challenging category is that in common parlance and in pop science, it is used to describe people with broader and milder symptoms. Indeed, one survey carried out by the reputable UK polling company YouGov declared that 29% of people in the UK are suffering from SAD[1]. This definitional ambiguity leads to a lot of confusion in the debate around SAD.
The second challenge is that research suggests that mechanisms underlying SAD vary between individuals [2]. With genetic, hormonal changes, disruptions in circadian rhythms and environmental influences all having been implicated, some question whether it is a conceptually useful grouping of independent conditions.
The only thing that is totally clear is that for most people who have been diagnosed (or self-diagnosed) with SAD, their symptoms significantly improve from exposure to more sunlight or bright artificial lights [3]. As such, rather than framing this discussion around SAD, a clearer evidential framework can be found by asking simply: how can bright light affect my body? And how can I use that to improve my wellbeing?
The Circadian Rhythm
The body has a daily internal cycle called the circadian rhythm. This cycle, characterised by daily variations in biological markers such as cortisol, glucose, body temperature, insulin, and melatonin, creates a temporal predisposition for resting and waking.
The advantage of having such an internal cycle is that it allows the body to "orchestrate physiological changes that lead, rather than lag, the daily change in environment" [4]. This means your body can start preparing you to wake up several hours before the sun begins to rise. The adaptive utility of having a circadian cycle has been demonstrated even in bacteria. Strains that have had their circadian cycle genetically removed have been shown to be outcompeted by those with a functioning rhythm [5].
Setting Your Clock
All clocks are founded on a process of regular oscillation to provide a singular unit of time. In a grandfather clock, this is provided by the pendulum; in modern watches, by quartz; while in atomic clocks, it is created by the vibration of electrons in atoms. The fundamental unit of oscillation of the circadian cycle derives from an intracellular production and degradation of proteins in the Suprachiasmatic Nucleus (SCN).
For the biological metronome occurring in the SCN to be of use to us, it must be set to the correct time of day. [This setting process is known as entrainment.] Exposure to light is by far the most important mechanism through which the circadian rhythm is entrained. Continuous entrainment is vital as the molecular oscillation produced in the SCN is typically longer than 24 hours. Therefore, just like a clock running fast, every day the cycle will drift slightly later compared to the time of day. Indeed, exactly this is observed in blind people who have no photonic inputs. [6]
Living Out of Sync with Your Circadian Rhythm
Living out of sync with your circadian rhythm is like driving a car in the wrong gear. It's both difficult and bad for you (/ the car). Acute effects of asynchrony with your circadian rhythm include increased anxiety[7] and impulsivity,[8] while also correlating to decreased cognitive and motor performance. [9] Many of these symptoms are familiar to anyone who's experienced jet lag. There is also evidence that chronic asynchrony can lead to increased risk of cancer, diabetes, and cardiovascular disease. [10] This asynchrony is also suspected as one of the key mechanisms involved in SAD. [11]
Circadian Light: Frequency, Brightness, Timing
In addition to rods and cones, there are special non-visual receptors in our eyes responsible for entraining our circadian rhythm called ipRGCs (intrinsically photosensitive retinal ganglion cells). [12] In this respect, the eye can be understood to be a dual sense organ with a distinct set of physiology being used to perceive both vision and environmental time. [13] Understanding how ipRGCs work is counterintuitive, as unlike the rest of our vision system, we have no direct conscious link to their workings. However, it is crucial for getting a sense of how light affects your body.
ipRGCs contain a photopigment called melanopsin which is responsible for their light-sensing ability. Melanopsin has a peak sensitivity of ~480nm, which means they are most strongly activated by blue light. Light of 460 nm (blue) wavelengths is twice as activating as 555 nm (green) light. [14]
The level of melanopsin in ipRGCs is relatively low compared to the level of photopigments in the rods and cones, which means that the melanopsin has a much lower likelihood of being hit by a photon. This low sensitivity, however, is counteracted by the ipRGCs' unique ability to aggregate photon exposure over several minutes. This aggregating, low-sensitivity characteristic is well adapted for sensing the gradual changes of light intensity that occur throughout the day.
Circadian Stimulus (CS) is a metric that quantifies the potency of light based on the effect it has on our ipRGCs and subsequent effect on our circadian rhythm. It does this by accounting for the coolness (through spectral distribution) and brightness of a light.[15] The maximum CS score is 0.7, which represents a saturation of circadian light input, while 0.1 is the threshold for circadian activation.
Even with high CS light, the circadian cycle only significantly responds to light at certain times. Exposure to light within a few hours before and after usual wake-up time has the effect of advancing the cycle—you will want to go to bed earlier. Conversely, exposure to light around usual bedtime delays the cycle—you will want to go to bed later. [10]
Between two hours after the usual wake-up time and two hours before bedtime, light exposure has minimal impact on moving the circadian rhythm. This variability in responsiveness to light is called the Phase Response Curve. The "delay zone" before night-time shows the time in which you can delay the cycle; the "advance zone" immediately following night-time shows the opposite. Exposure to light (or a lack of exposure) throughout the day has minimal effects on the circadian cycle.
To summarise: blueness, brightness, duration and timing of light exposure work together to determine its effect on the circadian rhythm. Now let's discuss how we can use this information.
Controlling Your Circadian Rhythm for Personal Gain
So You Want to Become a Morning Person?
I've always found that when I can wake up earlier and still get sufficient sleep, life becomes easier. My days feel longer, healthier and more productive. Indeed, there is evidence that being a naturally morning person improves your health, social and economic prospects.
Unfortunately, most people's (78%) circadian cycles tend to run slightly longer than 24 hours, [16] with men tending to have slightly longer (24.19 hour) cycles than women (24.09 hours). [17]As such, without any input from light, a typical person's cycle will gradually shift later in the day—making them inclined to stay up a little later every night. Exposure to high circadian stimulus light in the early morning advance-zone time helps prevent this phase.
Conversely, within two hours of your typical bedtime, exposure to high CS light will delay your circadian rhythm, shifting it later in the day. Importantly though, there is enormous variability between how sensitive people are to the effects of evening light, with one study finding a 50-fold variance in sensitivity.[18] [19] People who self-identify as night owls have been shown to be more likely to be sensitive to evening light and thus should be particularly cautious. [20]
Somewhat counterintuitively, recent studies have shown that bright light exposure in the day and 'early evening' can reduce some of the sleep-disruptive consequences of light exposure in the later evening.[21] What qualifies as 'early evening' will vary between individuals, but a conservative general rule is: maximise bright, warmer light exposure up to four hours before your usual bedtime. I recommend using bright slightly warmer light in the evening as most people find it more comfortable and natural during these hours.
What if I Already Am a Morning Person?
If you are someone who naturally already wakes up early, you are more likely to be one of the 22% of people who has a circadian cycle that is shorter than 24 hours.[22] For these people, the aforementioned advice of maximising morning light and reducing evening light may not apply. Probably because having a tendency to wake up early is something fewer people complain about, there is less research on light exposure interventions for these people. However, if you are struggling to stay up late enough, then the opposite principle of maximising evening light exposure may prove helpful.
Mitigating Jet Lag
These techniques can also be applied to crossing time zones. Let's say you're travelling from London to New York, a time zone difference of five hours. A few days before your departure, you can start 'delaying' your circadian cycle by exposing yourself to 30 minutes of high Circadian Stimulus (CS) light after sunset and shifting the time you wake up and go to bed back successively each day. It's best to prepare over several days, as the circadian cycle can be shifted by a maximum of one to two hours each day. With this adjustment, your circadian rhythm will be closer to the local time upon arrival, allowing you to stay up in the evening more naturally.
After a week in New York, your circadian rhythm will likely be fully adjusted to the local time. Upon returning to London, your body clock will now be delayed by five hours, making it most receptive to the phase-advancing effects of light five hours later than usual. Therefore, to adjust to London time as quickly as possible, it's recommended to expose yourself to 30 minutes of high Circadian Stimulus (CS) light from immediately upon waking, continuing until two hours after the time you would have been waking up in New York.
Making it Always Summer
In winter, the sun rises later and sets several hours earlier than in summer, which naturally affects our circadian rhythm. (In London, for example, at 51 degrees above the equator, this effect is significant: the longest day of the year is nearly nine hours longer than the shortest.) This leads to a change in the pattern of the circadian rhythm, dedicating more of the cycle to sleep and increasing the amount of REM sleep.[23] One study in Germany found that people slept one hour longer in December than in June.
In theory, then, CS light can be used after sunset in winter to mimic the longer daylight hours of a summer's day, allowing you to sleep less. I haven't been able to find any direct research on this intervention though and would advise experimenting with it cautiously.
In practice, maintaining a consistent daily pattern of bright light exposure can be challenging. This matters because irregular light exposure patterns lead to irregular circadian signals - much like having jet lag every day. Without consistent light exposure timing, your body clock receives conflicting signals causing your circadian rhythm to drift out of sync with your desired schedule. Research shows this misalignment disrupts sleep quality and can lead to the same negative effects we see with jet lag: poor concentration, mood changes, and fatigue. [7] Therefore, while using artificial light to extend your 'daylight' hours in winter is theoretically possible, it's only beneficial if you can maintain a very regular pattern of exposure.
Other Times You May Want to Control Your Circadian Rhythm
Beyond jet lag, understanding how to control your circadian rhythm can be valuable in other life situations. Night shift workers, for example, can use strategic bright light exposure to help their bodies adjust between day and night schedules, much like managing jet lag.[24] The timing and intensity of light exposure can help them maintain alertness during night shifts while still getting quality sleep during the day.
Another important application is in supporting adolescent sleep patterns. During teenage years, physiological changes naturally shift the circadian rhythm about two hours later. [25]This biological shift explains why teenagers typically struggle with early school start times - their bodies are literally programmed to stay up later and wake up later. Research has shown that carefully timed morning light exposure can help realign their circadian rhythms with school schedules, leading to improved sleep quality and better daytime functioning.[26]
The Immediate Effects of Light on Mood and Cognition
While we've discussed how light affects our circadian rhythm over time, light also has powerful immediate effects on our brain function, influencing mood, alertness, and cognitive performance within minutes of exposure through neural pathways distinct from the circadian system.
Bright Light and Alertness
The relationship between light intensity and alertness follows a logistic response curve: even moderate increases in brightness can significantly affect alertness. A typical well-lit office (100 lux) produces noticeable improvements, while very bright light (9,200 lux, similar to being outside on a cloudy day) can double this alerting effect. These effects are strongest in the early morning and evening, though sleep-deprived individuals remain sensitive to bright light's alerting effects throughout the day. [27]
Bluer light produces stronger and more immediate alerting effects, and when combined with high brightness, creates the most significant improvements in cognitive performance, enhancing everything from reaction time to decision-making speed and attention span.
The Calming Effects of Warm Light
In contrast, warmer light (around 2,800 Kelvin, similar to sunset) reduces activity in the amygdala, creating a calming effect.[28] Under warm light, people tend to perceive faces more positively,[29] partially explaining why we naturally prefer warmer lighting for evening relaxation and social situations.
These immediate effects complement the longer-term circadian impacts we discussed earlier. While circadian adjustment takes time, we can use different light qualities to optimise our mental state in the moment - bright, cool light for focus and alertness, warmer, dimmer light for relaxation and social situations.
Negotiating Light
There is considerable individual variability in how light is perceived. A person's mood affects how they respond to light;[30] being in a different stage of your circadian cycle will alter your lighting preference; [31] men tend to be more sensitive to the effects of blue light than women; [32] cataracts in older people make them less sensitive to light in general, and to blue light specifically. [33] There is also significant cultural influence on lighting preferences, with one study theorising that a preference for cooler light is found in people from places that experience more direct sunlight throughout the year. [34]
These variations in our physiology and preferences are important to recognise because more often than not, your interior lighting environment is shared with others. A common challenge is that one partner will be enjoying maximising their early morning bright, blue light while the other partner may find it too bright. One way to navigate this difference in preferences is to trade off colour temperature for brightness, as a high CS light can still be achieved at warmer colour temperatures by increasing the brightness. In effect, the blue light is being diluted by the warmer light, making it more palatable while maintaining its biological effectiveness.
Even in the daytime, many people express a preference for dimmer, warmer light in their homes. This is understandable given that the vast majority of bright, cooler light that people have experienced is low-quality LED or fluorescent lighting. These lights have low CRI (Colour Rendering Index), meaning that they do a poor job of mimicking the spectral distribution of sunlight. One question that needs more exploring is the degree to which exposure to higher CRI (more sunlight-like) light will lead to a change in people's assumptions around bright, bluer light. After all, few people are disappointed when the sun starts shining through their window on an overcast day.
Conclusion
Over the past decade, there has been increased awareness around the complex mechanisms through which diet, exercise, and sleep affect our wellbeing. These areas are too complex for one-size-fits-all advice, but understanding how they work empowers people to conduct informed personal experiments. They can try cutting out sugar, not eating before bed, or focusing on high-intensity interval training.
However, the basic understanding of how light affects our body remains largely confined to scientific research. Light exposure affects us through multiple, interrelated mechanisms: our circadian rhythm is entrained through specialised receptors called ipRGCs; we experience immediate effects on alertness and cognition and our bodies produce vital vitamin D through UV exposure.
Understanding these mechanisms, we can make informed decisions about our light exposure throughout the day and seasons. Whether dealing with jet lag, seasonal changes, shift work, or generally improving well-being, most people will be able to experience profound improvements to their quality of life through more intentional light exposure.
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