Understanding the Mysterious Science of Sleep

“Today I don’t feel like doing anything, I just wanna lay in my bed,” sings Bruno Mars in The Lazy Song. That is exactly what our inner narrative says every Monday morning, right? After the long weekend, having fun partying or catching up with some work, there is nothing worse than getting back into the weekday grind. All we want is an eternity of rest and sleep because – for the majority of us – sleep is a way to relax; it takes us away from the stressful reality of life.  

However, our physical condition when we sleep suggests that it is not actually very safe. When we sleep, we are in a mysterious state; we lie down and are vulnerable to predators without any defence. To minimise the dangers of sleeping, humans built houses that provide warmth and shelter from the weather and protection from predators. But sleeping is seen in various other lifeforms, not just us humans – and species that live in the wild experience conditions that are far more dangerous. Dreams are an even bigger mystery in the science of sleep; they do not seem to have any significant benefits, and their purpose is largely unknown. However, as with everything that is passed on from generation to generation, sleep and dreams must have a significant evolutionary advantage for our fitness and survival. 

Due to the different obstacles and routines faced by various species, different species sleep in different ways. Generally, predatory animals such as humans can sleep for long periods of time (1). Conversely, prey animals are constantly vigilant; instead of sleeping for a long time, they only rest for short periods (2). A particularly interesting example are dolphins and seals, who have evolved to keep half of their brain “asleep” while the other is “awake” during sleep (3). This shows us that sleep really is important for our survival, and that various organisms have even adopted mechanisms to combat obstacles to sleeping. So, the cost of sleeping must be worth it, right? The answer is “yes” – but scientists are unsure of exactly why. 


Why do we sleep? 
Various theories in literature on the purpose of sleep have been broadly categorised into two theories: the adaptive and restorative theories. 

One of the reasonings behind the adaptive theories proposes that creatures that are inactive at night have increased chances of survival due to a lower risk of injury (4). Another perspective suggests that humans sleep at night to conserve energy for the day, when it is more efficient to hunt for food (5). This theory has also been supported by the fact that humans have a 10 per cent decrease in metabolism during sleep (6). However, both theories were proposed in relation to our ancient lifestyle when we needed to physically hunt for food. Looking at our present lifestyle, this reasoning may not be as applicable – but it is still embedded in our system.

There are other theories that explore the reasoning behind sleep from the perspective of restoration. The restorative theory speculates that sleep allows us to repair cellular components that were used throughout the day, as many important growth hormones are shown to be released during sleep (7). This theory is also supported by the most widely accepted reasoning for why we sleep, which is that sleep is necessary for the growth and maintenance of the brain’s structure and function, and that it is crucial for optimising memory consolidation (8, 9). Sleep also affects other physiological aspects, such as immune function, endocrine function, cardiovascular health and mood (10, 11, 12) . Sleep disorders are shown to be associated with cardiovascular disease, and sleep reportedly enhances immune defences against pathogens. The fact that there are various theories explaining why we sleep shows that there is no single perfect explanation.

Regardless of why we sleep, we still get into bed at the end of the day. This is mainly because of our circadian rhythm, which controls our desire for sleep. Our circadian rhythm is controlled via the hypothalamus: an area at the centre of our brain that receives sensory inputs from various parts of the body. During sleep, the hypothalamus receives input from our eyes, which detect light levels (13). When we are exposed to high levels of light in the morning, the circadian rhythm promotes wakefulness (14). However, at night, when there is less exposure to light, the circadian rhythm promotes sleep due to the increase in the production of the sleep-regulating hormone, melatonin (15). 

Even though we have a central control system that regulates when we sleep, there is still a large variation in sleeping time among humans; some people sleep for only five hours, and others sleep for up to ten or more (16). Sleep duration is affected by factors such as physical and social environment, diet, activity, body mass index, comorbidities and mental health (17). Despite the contributions of lifestyle differences, some studies have shown that human sleep duration and timing is also influenced by genetic factors but is regulated by the circadian rhythm and brain activity (18). Currently, little is known about the specific genes and genetic mechanism involved in sleep duration, and more research is still being done in the area (19). These factors could explain why people often feel sleepy throughout the day, in addition to the variation in sleeping patterns in the population. However, as is so often the case in science, there is no one specific factor that may result in differences within the population – instead, a combination of these factors is likely to be responsible. 

The phases of sleep 
Did you know that there are different kinds of sleep? All humans go through two different sleep phases: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep (20). NREM takes up approximately 75–80 per cent of our total sleep duration, whereas REM takes up 20–25 per cent (21). Sleeping normally progresses from NREM 1–4 through to REM, and this cycle occurs four to five times each night (22) - for more details on sleep phases, check out Table 1!