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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