By observing the behaviour of all living beings, you would quickly realise that there are several shared physiological processes common among them. In fact, after just 24 hours you would notice that one of these behaviours is present in practically all animals: sleep

While sleeping may seem like a trivial event, it actually hides crucial information about our health, psychological condition, and habits. We all sleep, but very few people understand the science of it — the underlying mechanisms, composition, and factors that influence it.  

Thus, it’s time to answer the question: what is sleep?  

What is Sleep

From a medical and neuroscientific perspective, sleep is a temporary and recurring condition in which consciousness is partially or entirely suspended. During this statean individual’s integration with their surrounding environment is suspended. Meaning, if the body reacts to a stimulus, the responses are only partial, if any [1]. 

There are significant functional changes that accompany the state of sleep, these include: 

What are the Stages of Sleep? 

After understanding the physiological processes related to sleep, it is necessary to note its structure. 

Sleep is not a homogeneous event. In factit is made up of a series of so-called sleep cycles in which the brain goes through four distinct phases of sleep. On average, we cycle through the sleep phases 4-6 times each night. One entire cycle lasts around 90 minutes [2]. 

More broadly, sleep phases may be classified into two types: non-REM sleep and REM sleep. Their names come from the typical ocular characteristics of non-rapid eye movement (n-REM) and rapid eye movement (REM), respectively [2]. 

Sleep Architecture is the term used to describe the structural organisation of sleep. 

Chronologically, the phases occur in the following order: 

n-REM Sleep 


N1 is the first sleep phase. During this phase, we may easily wake up, our eyelids move more slowly, and our muscular activity becomes sluggish. This phase lasts 1 to 5 minutes and accounts for about 5% of the entire sleep cycle. 


Afterwards, we enter the N2 phase. During this phase, our body temperature and heart rate drop. This lasts around 25 minutes in the initial cycle and lengthens with each subsequent process. 


Then we have the N3 phase. This period is characterised by deep sleep. It is generally the most difficult to wake up during this time. Moreover, if we do wake up from this stage, we are likely to feel groggy and mentally impaired. Another effect of this stage is that our blood is redirected from the brain to the muscles, allowing them to receive oxygen and nutrients. Growth hormones are also produced, and our immune systems are strengthened. 

REM Sleep 

REM sleep is the last phase of the cycle. During this sleep stage, the brain is incredibly active. Recent studies have shown that some parts of the brain are even more active than when we are awake. Thus, this stage is called paradoxical sleep: the body is completely paralysed, but the brain appears awake. 

What are the Mechanisms of Sleep? 

Body Clock 

There are two co-operating systems that regulate our transition into sleep. 

The first of these systems is the internal biological clock, the innate time device of organisms.  

The second system is known as sleep-wake homeostasis. This is a fundamental concept for sleep regulation. A shortfall evokes an increase in the intensity and duration of sleep, while excessive sleep diminishes sleep inclination [2]. 

Circadian Rhythm 

These systems work on a neurobiological level through our circadian rhythms 

The term circadian comes from Latin and is comprised of the terms circa (approximately) and diem (day). In turn, circadian rhythms are defined as processes with a period length of circa 24 hours. 

The best-known circadian rhythm is the sleep-wake rhythm, which causes us to repeatedly switch between sleep and wakefulness.  

Other physiological parameters may also exhibit a rhythm with a 24-hour circa period. For example, our body temperature or the secretion of certain hormones fluctuate in a 24-hour rhythm [3]. 

This type of change is due to our synchronisation with the rotation of our planet. These movements cause periodic and recurring changes from brightness to darkness, warm and cold temperatures, and aspects such as the availability of food or lurking threats.  

Living organisms that adapt to such changes can optimise their ability to function and thus ensure their chances of survival. Therefore, circadian rhythms can be found in humans, animals, and plants alike. 

Circadian Pacemaker 

Circadian rhythms throughout the body are linked to a central circuit, sometimes called the circadian pacemaker [4]. This pacemaker is located in the suprachiasmatic nucleus (SCN) – part of the hypothalamus – just behind the root of your nose.  

The SCN is very sensitive to light, which is why light exposure highly influences our circadian rhythm. In the darkness, the SCN sends a corresponding signal to the pineal gland, which in turn produces, among others, the sleep-inducing hormone melatonin [2]. 

With this, the type of lights we are exposed to during the day, and especially before our sleep, can have a drastic influence on it [5]. 

In fact, disturbances to our circadian rhythm can have a significant influence on our ability to function correctly: 

  • Our hormone production, blood pressure, body temperature, or heart rate would no longer follow the sequence we have adapted over the years.  
  • Not only can this pose serious health risks, but it would also significantly reduce our well-being and productivity. 
  • If our internal clock no longer signals “night” to our body, it stays on high alert. Consequently, we will have difficulty falling asleep and repeatedly wake up during the night or early in the morning.
  • These events are accompanied by sleep fragmentation, lower sleep efficiency, and more insufficient quality sleep. 

Why Do We Need Sleep? 

By understanding these physiological processes, it is clear that sleep has a huge impact on our physiological and psychological well-being. 

The Functions of Sleep

Here are some crucial functions that occur as a result of sleep: 

  • Sleep serves a medicinal purpose by flushing out the toxins accumulating in the brain. For example, Beta-amyloid, a toxic protein that gets in the brains of Alzheimer’s patients, was washed away twice as fast in the brains of sleeping mice compared to mice awake [4] 
  • Sleep is also crucial for learningproblem-solving, and memory consolidation. A good night’s sleep is vital for academic success, as it has been shown to improve the examination results of college students.  
  • Sleep strengthens our immune system by enabling specialised immune cells, called T cells and Natural Killer cells, to work more efficiently fighting off infections. As such, insufficient sleep may jeopardise the effectiveness and potency of vaccines. In contrast, sufficient sleep after getting a vaccination boosts the production of T cells and Natural Killer cells to enhance our antibody response [4] 
  • Sleep is also essential for regulating the body’s metabolic and hormonal processes [4] 
  • In addition, REM sleep is generally crucial for emotional reactivity and emotional memory processing. During REM sleep, concentrations of the stress-related, anxiety-triggering chemical noradrenaline are shut off within the brain. Simultaneously, emotion and memory-related structures (i.e., amygdala and hippocampus) of the brain are reactivated during REM sleep. These systems enable the reprocessing of upsetting memories or experiences in a stress-free state. Therefore, REM sleep with a content-specific dream can help us dissolve the emotional charge from painful or traumatic episodes [6] [7]. 

How to Get a Good Night’s Sleep?

Although sleep is a physiological process, it is also a cultural and subjective phenomenon. Therefore, objective measures cannot completely guarantee ideal rest. However, there are some essential elements that can aid in improving sleep. 

Duration of Sleep 

One element that is undoubtedly related to the effectiveness of sleep is its length. The right amount of hours to sleep changes according to an individual’s age [8]. The American National Sleep Foundation has shown that it is possible to divide sleep times into nine age groups, with corresponding recommendations:

Sleep Hygeine

Concerning good sleep habits, a new word has been developed and utilised: sleep hygiene [9] [10]. The goal of sleep hygiene is to promote restful sleep through the implementation of better habits.  

  • It refers to a collection of behavioural and environmental suggestions initially recommended for people with insomnia.  
  • The advice given under the category of “sleep hygiene” often includes a broad range of potential actions that have been proven to aid sleep. 
  • These habits may become so ingrained in us that we do them nearly subconsciously without even realising it.  

However, individual sleep hygiene differs from person to person. That’s why we should experiment with different settings to see what works best for us [9] [10]. 

In the book titled “A Good Night’s Sleep”, Dr Lawrence Epstein, president of the American Academy of Sleep Medicine, illustrates a simple six-step plan to improve anyone’s sleep. This plan includes the following: 

1. Understand the significance of sleep.

Because sleep is crucial for optimal health, you should schedule enough time to receive the rest your body requires. For example, if you only get six or seven hours of sleep each night, any additional adjustments you make will be ineffective. 

2. Make a conscious effort to live a healthy lifestyle.

For example, try avoiding junk food, smoking, and drinking alcohol. Instead, find time to exercise and tire out your body.  

3. Develop excellent sleeping patterns.

Schedule your sleep habit and stick to it. This is an essential aspect of “sleep hygiene”. 

It is important to utilise your pre-sleep time to put any stress management methods you’ve learned – such as relaxation exercises, meditation, and biofeedback – into practice. Even though it’s tempting to stay in bed and watch TV, scroll social media, have unhealthy snacks, and so on – resist the urge.  

Furthermore, don’t take too many naps. This may cause you difficulty in falling asleep at night regularly. You should instead try to limit your sleep to one long overnight session. 

4. Maintain a consistent sleep/wake cycle.

A consistent sleep pattern maintains the circadian cycle by training the body to anticipate particular sleep and wake periods. Once you’ve figured out how much sleep you need, make a sleep routine and stick to it as closely as possible.  

5. Create a conducive sleeping environment.

For example, you can block electronic gadgets from your bedroom and use adjustable lighting to create an atmosphere that encourages sleep hygiene.  

6. Be on the lookout for sleep saboteurs.

Aside from this, other environmental habits include making your bed, placing aromatic scents in your bedroom, and washing your linens and pillows every two weeks.  

The helpful idea is to avoid naps too close to your usual bedtime. Also, pay attention to the use of artificial light. You may expose yourself to it during the daytime but try to minimise your exposure to it in the evening.   


As previously described, light is vital to the circadian rhythm and the primary synchroniser of our internal clock. This is because it affects the production of melatonin, the sleep-inducing hormone that signals night to the body [5]. 

The suppression of melatonin by natural light is intended and called for during the day as it promotes attention, alertness, and mood [5]. At the onset of darkness, melatonin production slowly increases. It stays elevated for the course of an approximately twelve-hour period. 

However, artificial light can disrupt this system. 

A good example of this is an experiment conducted by Kenneth P. Wright and his team, who discovered a paradox concerning the use of electric light. Many markers of the internal circadian rhythm appear delayed by a couple of hours because of artificial light. This delay also occurs in melatonin production.  

When we eliminate the exposure to artificial light, the melatonin offset occurs more than 50 minutes before we wake up. This temporal shift is fundamental because it would anticipate when our cognitive faculties are at their lowest.  

In other wordsthrough the elimination of artificial light exposure, we would have the ability to wake up with our cognitive faculties boosted [11]. Therefore, we would not experience the typical difficulties of getting out of bed. Instead, we would feel deeply rested and ready to start a new day. 


Temperatures can affect our sleep quality, circadian rhythm, and the beneficial effects of rest.  

As previously explained, our bodies incur rhythmical changes. These rhythmical changes include a drop in body temperature, signalling sleep time and characterising the NREM sleep phase.  

More importantly, when we enter the REM stage, both the behavioural and autonomic thermoregulation are suspended [3]. During this phase, our body homeostasis stops appropriately functioning.  

In other words: 

  • If we are in a hot environment, our body cannot properly balance the inside-outside temperature. This leads to an overall more inferior sleep quality during summer compared to other seasons.  
  • However, people exposed to a temperature too low during the first sleep stages reported difficulties falling asleep (i.e., longer time) and inferior sleep quality [3] 
  • One study has compared the differences between sleep phases on two different mattresses: a conventional low-heat capacity mattress versus a high-heat capacity. The results show that sleeping on a high-heat capacity mattress led to a more significant decline in core body temperature and an overall reduced heart rate. These effects were translated into a considerable increase in the deep sleep stage [3]. 

Sleep Deprivation and Disorders

Only when we stay up more than necessary do we truly recognize the importance of sleep. Anyone who has had a sleepless night or been unable to sleep for any number of reasons knows the typical consequences: difficulty concentrating, tiredness, negative mood, and fatigue [10]. 

Physical Health

Lack of adequate sleep can also cause more severe effects on a person’s health: 

  • Individuals who sleep less than seven hours per night are more likely to have higher average body mass indexes and develop obesity than those who slept more. 
  • Lack of sleep suppresses the immune system. People who sleep six hours a night or less are four times more likely to catch a cold when exposed to a virus than those who sleep more than seven hours a night [4]. 
  • Inadequate sleep can cause many diseases, including cancer, hypertension, coronary diseases, stroke, depression, and diabetes [4]. 
  • Sleeping six hours per night can also increase your risk of having a car crash by 33% compared to sleeping seven or eight hours a night [12]. 

Mental Health

The effects of a good night’s sleep are not only physical. There is also a great connection between sleep deprivation and mental health based on studies: 

  • Optimal sleepers experienced significantly fewer symptoms of depression and anxiety as well as considerably higher levels of environmental mastery, personal growth, positive relations with others, purpose in life, and self-acceptance [6] [7]. 
  • A study on young adolescents aged 14-15 showed a negative relation between sleep at night and experienced levels of anxiety, depressive feelings, and fatigue during the following day [6] [7]. 


As we have seen, sleep is a complex phenomenon. It is understood as both a social and cultural phenomenon as well as a physiological and natural event. Therefore, it is essential to emphasise that, although scientific, psychological, physiological, and neuroscientific research is taking critical steps to understand sleep, it remains a subjective phenomenon. For this reason, it is crucial to rely on the assessment of experts who can understand your needs and the causes of possible poor sleep.  



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