Home » Understanding Sleep » How Is Sleep Regulated? The Two-Process Model

How Is Sleep Regulated? The Two-Process Model


You have probably noticed that from the time you wake up, your energy level seems to grow and then start falling down again – the longer we are awake, the more tired we feel. This can be ‘fixed’ only by sleeping. Then again, there are days when we are sleepy all day because we haven’t had enough sleep the previous night.

Our activity time and rest time, alertness and sleepiness are regulated by chemicals – hormones, neurotransmitters, but also many more factors. Scientists have found two main processes that dictate our sleep-wake cycle. They relate and partly depend on each other.

These processes are called the circadian process and sleep-wake homeostasis.

The circadian process – process C

‘Circadian’ means ‘around the day’, and the circadian rhythm is a term used to describe a rhythmic activity of the processes in our body. The circadian rhythm mainly depends on two things: our inner biological clock (what we’re used to) and the day/night time (what the actual time is).

It dictates hormone release, and they tell us when we’re hungry, sleepy, or energetic. It also regulates our body temperature (which is higher when we are more alert and lower when we’re sleepy), brainwave activity, and more.

The presence or absence of the daylight sends cues to our body. However, when we’re suffering from jet lag (when we’ve changed the time zone and still aren’t accustomed to the new time) our circadian clock keeps ticking in tune with the old time zone for some time until it gets adjusted.

When we talk about the two-process model, the circadian process is called process C.

To sum up, process C regulates many body functions throughout the day, including wake and sleep patterns.

The sleep-wake homeostasis – process S

‘Homeostasis’ literally means ‘same state’. Homeostasis a word is used to describe a balance in the body which is maintained by certain processes. For example, temperature homeostasis is the state in which our body preserves the same body temperature (normally with very small fluctuations).

The sleep-wake homeostasis refers to our body’s inclination towards sleep after we’ve been awake for some time. The longer we’re awake, the stronger our need for sleep. As certain chemical substances accumulate in our brains, we feel more tired until finally, we decide to sleep.

The sleep-wake homeostasis is called process S, and it works like a timer – after a certain number of hours, it reminds the body that it’s time for sleep.

An interesting fact is that sleep itself doesn’t affect the homeostatic regulation of the sleep drive. Only one sleep stage, the slow-wave sleep (SWS), also called deep sleep, is able to diminish our need for sleep. This is why if we take a short nap during the day, we will go to bed at our usual time that night, but if we take a nap of over 60 minutes, we reach deep sleep. This means that in the evening we will not be as tired because we have ‘written off’ some of our sleep drive.

How process C and process S work together

These two processes make a well-balanced, healthy rhythm. They affect each other and create our sleep-wake cycle. EEG recordings of brain waves from the hypothalamus, or, more precisely, from the suprachiasmatic nuclei (SCN), show that processes C and S ‘interact continuously’. SCN is found be the main circadian rhythm regulator.

When we’re tired late at night, process S – our sleep drive – is the strongest. It is boosted by the circadian release of melatonin. Melatonin is a hormone that’s produced at night and makes us sleepy. Therefore, at night, both processes work towards getting us to sleep.

Over the night, process S dissipates, and in the morning, it is ruled over by process C, which wakes us up. Our circadian system detects morning light, stops melatonin production and begins producing ‘alertness hormones’, like cortisol.

Once awake (after we shake off sleep inertia), our sleep drive is the lowest. We enjoy full alertness and productivity. Throughout the day, our sleep drive increases, but it is influenced by the circadian rhythm which keeps us awake until the time when melatonin is secreted again. This ‘opens the sleep window’, which means we’re comfortable with sleeping at about that time.

Sleep process diagram
Figure 1. Processes C and S over the course of two days. Image source: Walker, Why We Sleep?

The longest distance between the circadian rhythm and sleep drive is when we are the most sleepy, whereas the shortest is when we are the most alert. Note that Figure 1 is just a model and that our personal chronotype (sleep-wake preference) might be different.

This means that when process S (sleep drive) is the strongest, and process C (alertness drive) the weakest, we tend to sleep. In the opposite case, we are awake and fully alert.

One of the things that make us sleepy is the buildup of adenosine. Adenosine is a neurotransmitter which accumulates as a result of the nerve cell activity. It is ‘cleared up’ from the brain after good, deep sleep. This is why poor quality sleep results in daytime sleepiness.

Caffeine can block out the effect of adenosine, making us more alert, but it doesn’t reduce the overall need for sleep.

What if process S and process C don’t match?

When these two processes are well-aligned, you are able to get enough sleep and be fully rested throughout the day. When they are not, you might suffer from social jet lag – the state where your sleep-wake time is different from that of society. You might go to bed too late and wake up too late, which makes it difficult to do your daily tasks.

Another problem that could arise is an inability to fall asleep. You might be very tired, but a have problem falling asleep (maybe as a consequence of LED lights which trick your circadian rhythm), so even though process S is pushing you to sleep, process C might delay the sleep onset, as if saying ‘it’s still daytime – no melatonin and no sleep for a couple more hours’.

Sleep processes throughout the day diagram
Figure 2. How sleep-wake homeostasis (process S) and the circadian rhythm (process C) work throughout the day. Image source: Walker, Why We Sleep?

When we sleep well, our C and S processes are well-aligned. However, if we were to pull an ‘all-nighter’ plus a day without naps, our circadian rhythm would most likely remain normal, especially if we keep exposing ourselves to the natural light. Our sleep urge would keep growing and the effect of adenosine would be stronger with every hour.

The morning/afternoon after the all-nighter, we may feel less tired, as a consequence of our circadian rhythm telling us that it’s time for alertness. In the evening, on the other hand, we’d feel an extremely high sleep urge and might as a result sleep for many hours – well over 8 – in order to repay some of the accumulated sleep debt.

Recent research has shown that a genetic mutation might cause a person to be an early bird or a night owl, however, this shouldn’t be an excuse for most of us. Our chronotype is largely flexible, and we can easily adjust our sleep time so that we sleep somewhere between 11-12pm and 8-9am.

How to get the optimal sleep-wake pattern?

In order to ‘fix’ our circadian rhythm, we should mind the light as it has an immense effect on our mood, health, and sleep. Just follow the natural light exposure – get plenty of sunlight during the day and avoid screens and LED lights at night.

However, light is not the only thing that impacts our circadian rhythm – there is food, drinking, going out and staying up late, working in shifts, and temperature, among others.

Sleeping enough at night and getting the sufficient amount of deep sleep will help with our sleep homeostasis. Deep sleep can be lost if we are under a lot of stress, take certain drugs, drink alcohol (it’s highly recommended to avoid the nightcap), or have an untreated sleep disorder.

Keep a regular sleep schedule and stick to good sleep habits; which include avoiding technology use, strong food, vigorous exercise and caffeine at night; keeping the room cool, dark and quiet and, in the morning – getting up right away, without the snooze button.

Additional resources

  1. Borbély A. A, Daan S, et al.The two-process model of sleep regulation: a reappraisal. Journal of Sleep Research. January 14, 2016. https://www.ncbi.nlm.nih.gov/pubmed/26762182 Accessed February 2, 2019.
  2. Walker M. Why We Sleep: Unlocking The Power Of Sleep and Dreams. Published October 3, 2017. Simon and Schuster. ISBN: 1501144316

The information on this website is not intended to replace a one-on-one relationship with a qualified health care professional and is not intended as medical advice. Read our full medical disclaimer.