In the quest to become more effective and productive, sleep is an enormously important process to optimize. Most of us spend (or at least think we should spend) 7.5 to 8.5 hours in bed every night, a third of a 24 hour day. Not sleeping well and not sleeping sufficiently have known and large drawbacks, including decreased attention, greater irritability, depressed immune function, and generally weakened cognitive ability. If you’re looking for more time, either for subjective life-extension, or so that you can get more done in a day, taking steps to sleep most efficiently, so as to not spend more than the required amount of time in bed and to get the full benefit of the rest, is of high value.

Understanding the inner mechanisms of this process, can let us work around them. Sleep, baffling as it is (and it is extremely baffling), is not a black box. Knowing how it works, you can organize your behavior to accommodate the world as it is, just as taking advantage of the principles of aerodynamics, thrust, and lift, enables one to build an airplane.

The most important thing to know about sleep and wakefulness is that it is the result of a dual process: how alert a person feels is determined by two different and opposite functions. The first is termed  the homeostatic sleep drive (also, homeostatic drive, sleep load, sleep pressure, and process S), which is determined solely by how long it has been since an individual has last slept fully. The longer he/she’s been awake, the greater his/her sleep drive. It is the brain's biological need to sleep. Just as sufficient need for calories produces hunger, sufficient sleep-drive produces sleepiness. Sleeping decreases sleep drive, and sleep drive drops faster (when sleeping) then it rises (when awake).

Neuroscience is complicated, but it seems the chemical correlate of sleep drive is the build-up of adenosine in the basal forebrain and this is used as the brain’s internal measure of how badly one needs sleep.1 (Caffeine makes us feel alert by competing with adenosine for bonding sites and thereby inhibiting reuptake.)

This is only half the story, however. Adenosine levels are much higher (and sleep drive correspondingly lower) in the evening, when one has been awake for a while, than in the middle of the night, when one has just slept for several hours. If sleepiness were only determined by sleep drive, you would have a much more fragmented sleep: sleeping several times during the day, and waking up several times during the night. Instead, humans typically stay awake through the day, and sleep through the whole night. This is due to the second influence on wakefulness: the circadian alerting signal.

For most of human history, there was little that could be done at night. Darkness made it much more difficult to hunt or gather than it was during day. Given that the brain requires some fraction of the nychthemeron (meaning a 24-hour period) asleep, it is evolutionarily preferable to concentrate that fraction of of the nychthemeron in the nighttime, freeing the day to do other things. For this reason, there is also a cyclical component to one’s alertness: independent of how long it has been since an individual has slept, there will be times in the nychthemeron when he/she will feel more or less tired.   

Roughly, the circadian alerting signal (also known as process C) counters the sleep-drive, so that as sleep drive builds up during the day, alertness stays constant, and as sleep drive increases over the course of the night, the individual will stay asleep.

The alerting signal is synchronized to circadian rhythms, which are in turn attuned to light exposure. The circadian clock is set so that the alerting signal begins to increase again (after a night of sleep) at the time when the optic nerve is first exposed to light in the morning (or rather, when the the optic nerve has habitually been first exposed to light, since it takes up to a week to reset circadian rhythms), and increases with the sleep drive until about 14 hours later (from the point that the alerting signal started rising).

This is why if you pull an “all-nighter” you might find it difficult to fall asleep during the following day, even if you feel exhausted. Your sleep drive is high, but the alerting signal is triggering wakefulness, which makes it hard to fall asleep.

For unknown reasons, there is a dip in the circadian alerting about 8 hours after the beginning of the cycle. This is why people sometimes experience that “2:30 feeling.” This is also the time at which biphasic cultures typically have an afternoon siesta. This is useful to know, because this is the best time to take a nap if you want to make up sleep missed the night before.

The neurochemistry of the circadian alerting signal is more complex than that of the sleep drive, but one of the key chemicals of process C is melatonin, which is secreted by the pineal gland about 12 hours after the start of the circadian cycle (two hours before habitual bedtime). It is mildly sleep-inducing.

This is why taking melatonin tablets before is recommended by gwern and others. I second this recommendation. Though not FDA-approved, there seem to be little in the way of negative side effects and they make it much easier to fall asleep.

The natural release of melatonin is inhibited by light, and in particular blue light (which is why it is beneficial applications to red-shift the light of their computer screens, like flux or reds.shift, or wear red-tinted goggles, before bed). By limiting light exposure in the late evening you allow natural melatonin secretion, which both stimulates sleep and prevents the circadian clock from shifting (which would make it even more difficult to fall asleep the following night). Recent studies have shown bright screens ant night do demonstrably disrupt sleep.2

The thing that interests me about this fact that alertness is controlled by both process S and process C, is that it may be possible to modulate each of those processes independently. It would be enormously useful to be able to “turn off” the circadian alerting signal on demand, so that a person can fall asleep at any time off the day, to make up sleep loss whenever is convenient. Instead of accommodating circadian rhythms when scheduling, we could adjust the circadian effect to better fit our lives. When you know you’ll need to be awake all night, for instance, you could turn off the alerting signal around midday and sleep until your sleep drive is reset. In fact, is suspect that those people who are able to live successfully on a polyphasic sleep schedule get the benefits by retraining the circadian influence. In the coming posts, I want to outline a few of the possibilities and (significant) problems in that direction. 

1 ^{Blanco-Centurion, C., Xu, M., Murillo-Rodriguez, E., Gerashchenko, D., Shiromani, A., Salin-Pascual, Shiromani, P. (2006). Adenosine and Sleep Homeostasis in the Basal Forebrain. Journal of Neuroscience, 8092-8100.}    

2National Sleep Foundation. (2011, March 7). Annual Sleep in America Poll Exploring Connections with Communications Technology Use and Sleep. Retrieved August 17, 2011, from http://www.sleepfoundation.org/article/press-release/annual-sleep-america-poll- exploring-connections-communications-technology-use-.