sleeping illustration

How does the brain know when to fall asleep and when to wake up? One brain, two mechanisms, several neurotransmitters, and the effect of light explained.

Even if we skip archaic mythical explanations, there are two stories explaining this phenomenon. The first, an older model of two systems as described in 1982 and the second, a never-ending narrative of researching all chemicals involved in the process.

Basic model explained: The astonishing simplicity of sleep & wake mechanisms

Many people don’t like models, I know, but I have never met anyone who would not fall for the beautiful interaction of the two sleep systems described in 1982. They are so obvious!

The first system is simply called “S”, standing for the sleep drive, and it simply means that the more tired you are, the more you want to sleep.

Throughout the day, our brains pile up a substance called adenosine, something like a measure of exhaustion. The more active you are, and the more adenosine is produced, the more tired you feel afterwards. In sleep, adenosine gets dissolved so that we wake up refreshed. This is exactly how coffee works, by the way: caffeine blocks the effects of adenosine, making us feel and act fresher than we are.

However, this “S” system with adenosine would put us to sleep immediately after we get tired. This is not what we want. Instead, we want to enjoy our evening after finishing hard work despite our brains being flooded by adenosine, right?

Evolution was smart enough to expect that we might still want to wait for a certain hour before falling asleep and so provided us with a second system, named circadian or simply “C”. No matter how tired you are, the circadian system makes you sleepy only in the late evening.

The key molecule of this system is called melatonin, a hormone that puts our brain into a sleepy state and is only produced when we are not exposed to light. Crucially, it is the blue light we must avoid because it makes our brain think there is a bright, sunshiny day out there; this is why self-help articles and professionals suggest turning off the screens in your life before you plan on falling asleep. Normally, its production peaks in the middle of the night and then starts decreasing again in a natural cycle. This cycle may be further promoted or impaired by light.

Sleep regulation = C + S

Putting both systems together, we can finally see the sleep pattern. We wake up in the morning refreshed with some residual amounts of melatonin but very low levels of adenosine. Even though we gradually build higher adenosine levels throughout the day, we remain awake because we produce no melatonin at all. It is only in the evening when melatonin production kicks in (adding to the already high adenosine levels) that we feel especially tired and our body begins preparing for sleep.

Iit is also thanks to these two overlapping systems that we remain asleep during the night. In the evening, our melatonin level does not reach its peak. When combined with the enormous adenosine levels, however, it is enough for us to fall asleep. During the night, adenosine is decreasing so melatonin peaks in order to preserve our sleep. Plus, there is a so called “sleep switch” maintaining our sleep which allows us to continue sleeping into the morning despite both melatonin and adenosine plummeting.

These smart mechanisms have profound implications for sleep hygiene. In order for us to sleep well, we need to be active during the day so that we pile up a decent amount of adenosine. In the evening, we should refrain from bright light (including the blueish irradiance of our screens) which disrupts melatonin production. We should also avoid coffee for several hours before bed time. In contrast, we should boost our light exposure as much as we can during the day in order to suppress melatonin production efficiently.

The ticking genes

The circadian or “C” system is not only regulated by light but through genetic processes in the cells of our “inner clock” which can measure time quite well. That’s why regularity in sleep patterns can improve sleep quality. This is also why morning bright light can help with falling asleep some 16 hours later: it assists the maintenance of our sleep patterns.

There are so called Clock genes ticking endlessly within the centre of our brain’s so called clock machine, the suprachiasmatic nucleus. They are carefully developed by evolutionary pressures not to slow down or speed up in different temperatures. It is their timekeeping which makes us preserve our routine even during dark days, winter, or in professions where natural light is not readily available (miners, for example). However, we still need the external light to regulate this system because it has not been built to regulate itself. If you’re curious, this is why the day-cycle for miners living deep underground extends from a 24 to 25 hour day-cycle; the body cannot maintain the normal cycle without light.

Our brain’s time-measures were developed in order to constantly adapt to the external light fluctuations in day and night as well as seasonally. The bad news is that this adaptation makes us vulnerable to unnatural lights of lamps or displays as well as unnatural darkness of lives lived indoors. The good news is that we can use this system for therapeutic purposes. Strong light exposure early in the morning tells the system it is bright day already. The brain’s mechanisms easily deduce that the inner clock mechanism is likely to be delayed. Therefore, with early bright light or sunshine exposure, the brain time-centre in the suprachiasmatic nucleus gradually starts advancing the onset of melatonin production. As a result, we are sleepy earlier and wake up more easily.

The subtle binds that make our brains work

Second, there are many more switches and fuses. Having only the S and C system, we would be unable to wake up refreshed early in winter mornings or to stay up when taking care of a child or elderly relative. We can still have a nap after lunch or party the whole night without feeling tired, contrary to what the S and C systems alone would predict. Luckily, our sleep & wake regulation is of much more elaborate design.

For example, our arousal is supported by fluctuations of cortisol nicknamed “the stress hormone” – which is actually responsible for more than mere stress. It naturally peaks in the morning (a smart mechanism to make us aroused from the beginning of the day) when the C system is not fully switched on yet. This hormone increases after eating, smoking, and drinking coffee. Hence, these activities are not recommended before bed time. It also allows us to stay awake and refreshed at parties or during work.

There are many more systems and molecules balancing our arousal. Besides cortisol, the monoamines (serotonin, norepinephrine, etc.) in particular increase activations of our brains and bodies, thus linking disturbances of the sleep-wake cycle to depressions. There are feedback loops between brain centres mutually inhibiting each other which also help us stay asleep or remain awake. There are even more subtle mechanisms optimizing alternation of different sleep stages.

As Alexander Borbely, the author of the famous S & C system, puts it in his recent review:

“Our view of the circadian system has undergone profound changes. The SCN is now viewed as orchestrating and integrating rhythms rather than simply generating and driving them. (…) This research has yielded novel, non-pharmacological antidepressant treatments such as light therapy, sleep deprivation, and sleep phase advance.”

Borberly et al. (2016). The two-process model of sleep regulation: a reappraisal. J Sleep Res. 25, 131–143 .

For more information on the interplay among many brain substances regulating our sleep and wake times, arousal, energy, mood, and motivation, open our next article from the Science behind series: