At the start of a new day, we are full of pep—alert, focused and full of energy. As the morning marches on, this gives way to the yawning and temple rubbing which often accompanies mid-afternoon sluggishness. And by the time evening rolls around, bed draws us in like a tractor beam as everything in our body seems to be telling us it’s time to lie down.

But why does this happen? How does our body know when we need rest and more importantly, how does it let us know?

The answers to these questions are rooted in the sleep-homeostatic process, or sleep-wake homeostasis that our body maintains. Here we’ll learn how this mechanism functions, and how you can use it to achieve the best slumber possible.

What it does

To understand this complex process, it is helpful to think about the word homeostasis itself. In biology, this term refers to the idea of keeping parts in balance. In the human body, we have many homeostatic systems that help us achieve ideal, balanced conditions such as consistent body temperature.

The sleep-homeostasis process, then, is simply the way that our body tries to keep the amount of Zzzs we are getting in balance with the amount that we need.

This process is one part of a two-part model of sleep regulation first proposed by the Swiss scientist Alexander Borbély in 1982. Along with the internal clock of the body, known as the circadian system, Borbély claimed that the sleep-homeostasis process works to keep track of when we need to sleep, and pushes us towards rest when the time is right. Through experimental analyses of brain activity via EEG, Borbély determined that this process, which he called “Process S,” worked in a cyclical manner throughout the course of an entire day of rest and activity.

Further studies confirmed his general hypothesis and cemented our understanding of the sleep-homeostasis process as the main mechanism by which our body drives us towards rest.

How it works

Like the way our body maintains homeostasis of temperature, sleep-homeostasis is a process of sending, receiving and interpreting biological signals.

In a nutshell, the process can be explained through the concept of “sleep pressure.” This refers to the biological forces that cause us to feel sleepy, and they are at the heart of this homeostatic process. As one review of the science surrounding this system puts it,

“The sleep homeostatic process can be basically described as a rise of sleep pressure during wakefulness and its dissipation during sleep…The term ‘homeostasis’ refers to the compensatory facilitation of deep, continuous, and long sleep episodes when sleep is initiated after a long episode of wakefulness.”

The pressure that calls us to our beds is exerted through the manipulation of neurological substances in the brain. Certain neurochemicals, referred to as “sleep factors” by researchers, build up in our brains throughout the course of the day, and are dissipated as we sleep. The more of these factors we have built up, the more pressure is exerted upon us.

Known sleep factors include adenosine, BDNF, PGD2, and TNFα. These neurochemicals have been linked to immune functioning, neural plasticity, and regulation of energy metabolism. While not fully understood, several theories outline possible ways that sleep factor buildup in our brain over the course of the day may exert sleep pressure as a way to regulate these important bodily functions.

In any case, when there are sleep factors built up in the brain, such as in the evening after a long day, the physical feeling of sleepiness sets in. Our body reacts to the need to eliminate these substances by calling us to snooze.

Adenosine in particular seems especially important in inducing feelings of drowsiness. Caffeine, which is relied upon as one of the world’s most popular sleepiness fighters, works by blocking adenosine from building up in the brain.

Working with the circadian system

Sleep-homeostasis systems function around the arbitrary times when we sleep and when we are awake. That is, sleep pressure is exerted more and more while we are awake, and dissipates when we sleep regardless of when these periods occur.

However, the second part of Borbély’s regulation theory highlights the circadian system as an important tool for aligning our sleep-wake cycle with the natural day and night cycle of the earth. If you want to learn more about how the circadian system regulates when we feel sleepy, check out our article on the subject here.

Without the circadian system to inform us about when is an appropriate time to doze of and when it isn’t, we would likely give in to the sleep pressures we experience throughout the day. Together, these two systems work to ensure that there is natural pressure for us to sleep the right amount, at the right time.

Make the process work for you

Based on the functioning of the sleep-wake homeostasis process, here are a few quick tips for getting better sleep:

• Pay attention to your body: There are hundreds of thousands of years of evolution behind our body’s ability to let us know when we need rest. Take advantage of that. As much as you can, go to bed when you feel sleepy. And when you feel like you might need a bit more rest than usual, do not hesitate to carve out time to satisfy that urge. Putting off needed sleep is not a good idea.
• Keep a consistent schedule: The more you keep your bedtime and rising time consistent, the more your body will naturally adjust to that schedule. It will be much easier to get restful slumber when your rhythms are consistent.
• Avoid lots of caffeine in the afternoons and evenings: While caffeine can help stave off feelings of sleepiness, it might also contribute to problems maintaining a healthy circadian cycle.
• Try napping: During the middle of the day (about 1pm for early risers, and closer to 3pm for late risers) you will generally feel the pull of sleep pressure as sleep factors begin to build up. Rather than go for a cup of coffee or simply push through the feeling, try taking a power nap to restore some of your energy.