The extraordinary powers of sleep

The virtues of sleep are unparalleled and probably too little known to the general public. Duration, room temperature, and many other factors will influence its effects. And what are these effects? How does our body strengthen itself? In what way can we regain our energy by simply closing our eyes to dive into our unconscious? Let's explore together the extraordinary powers of sleep!

The sleep phases

Once again, the body reveals itself to be a tremendously well-designed machine! Each of us has an internal biological clock that allows us to feel the need for sleep and even unconsciously know when it's time to wake up (which may not necessarily coincide with the schedules of our active lives – the alarm clock is a fantastic invention!). The initial signals guiding us towards sleep (yawning, drooping and stinging eyes, slight shivers) are regulated by a complex system known as the circadian system, unique to each individual, which regulates the wake/sleep alternation over approximately 24 hours. This rhythm is in turn regulated by the expression of so-called "clock" genes, the synthesis of the sleep hormone called melatonin, and specific cells in the eye located in the retina (these are melanopsin-containing ganglion cells, which are particularly sensitive to the blue light emitted by screens. Perceived as a signal of daytime by these cells, it can hinder properly falling asleep). Contrary to what one might think, we do not sleep uniformly throughout the night. After the initial phase of falling asleep, 3 to 6 sleep cycles will follow, with each cycle lasting 90 minutes to 2 hours (Figure 1). And during each cycle, two types of sleep will alternate:

Fig. 1 : Stage of sleep in adults (https://www.simplypsychology.org/sleep-stages.html)

- Slow-wave sleep, which leads us into deep sleep with partial muscle relaxation. During this phase, the body consumes less oxygen and slows down its metabolism.

- REM (Rapid Eye Movement) sleep, which resembles the wakeful state due to high brain activity. Another frequent characteristic is rapid eye movements. Muscles are completely relaxed. This is the stage of intense dreams that are most often remembered upon waking up.

Sleep on it!

This saying is indeed true! Sleep is crucial for the brain, and this holds true on multiple levels. During the first months of its life, a baby sleeps a lot. Unlike adults, newborns have sleep cycles consisting of approximately 60% REM (Rapid Eye Movement) sleep, which is then referred to as restless sleep. This high proportion of REM sleep is essential for the maturation of the baby's brain. New connections are formed, and others are lost. The baby's sleep cycles are also shorter than those of adults, lasting about half as long, or approximately 60 minutes. As the child grows, they will gradually need less sleep, especially less REM sleep and slow-wave sleep (Figure 2).

Fig. 2 : Differences of stages of sleep between adults and newborn (https://lullabyandme.com/blog/f/how-infant-sleep-is-different-from-adult-sleep)

From the very beginning of life, sleep promotes memory and learning. Two areas of the brain communicate through specific waves during slow-wave sleep. The hippocampus, which has acquired information during the day, transmits it to the neocortex, which integrates it more slowly and durably into our brain. Sleep, in this way, consolidates the learning acquired during the day and strengthens the ability to assimilate new learning the next day.

He who sleeps, dines

Sleep provides both mental and physical recovery for our body. Each of the two sleep phases plays a crucial role in these processes.

- Slow-wave sleep primarily facilitates physical recovery. As we have seen, this phase leads to a decrease in oxygen consumption by various organs and a reduction in energy consumption by metabolism. This slowing down of metabolism, in particular, results in a decrease in body temperature, which hovers around 36°C. Studies have shown that the poorer the sleep on one night, the more intense slow-wave sleep tends to be the following night.

- REM (Rapid Eye Movement) sleep has a role to play in recovering from mental fatigue. This is paradoxical in the sense that these are periods of sleep during which the brain is most active. The mechanisms of this recovery are not well understood, but it has been shown that exposure to stress increases the time spent in REM sleep at the expense of slow-wave sleep. Genetic regulation of signals related to cell death and survival in the brain may be involved in this process. Similarly, this recovery seems to be linked to the regulation of cortisol (stress hormone) by REM sleep.

Early to bed and early to rise (but not too much!) makes a man healthy, wealthy, and wise

All phenomena promoting recovery are essential for the proper functioning and health of the body during waking phases. For example, sleep is crucial for the regulation of appetite hormones (it decreases ghrelin, which stimulates hunger, and increases leptin, which has the opposite effect). Similarly, hormones regulating glucose metabolism (cortisol and growth hormone) as well as pancreatic sensitivity to glucose are disrupted by lack of sleep. It has been shown that insufficient sleep increases the risk of obesity, type 2 diabetes, as well as high blood pressure and metabolic syndrome (a syndrome that combines at least 3 of the following symptoms: hyperglycemia, hypertriglyceridemia, high blood pressure, and excess fat around abdominal organs). Therefore, not getting enough sleep is not without consequences.

To fall into the arms of Morpheus for a better immunity

Even if the precise mechanisms are not yet fully understood, it has been proven that lack of sleep increases certain molecules of the immune system (from the cytokine family) that have pro-inflammatory effects. The cause-and-effect relationship is not yet clear, but the effect is evident. This observation applies to adults as well as children. It can lead to more serious consequences since the accumulation of molecules promoting inflammation from a young age increases the risk of future health problems.

Sleep is a precious asset that allows us to recover from physical and mental fatigue and, importantly, to maintain long-term health. Numerous studies are still ongoing to better understand sleep and make it our best ally. For example, a team in the United States recently attempted to highlight the link between late awakening and psychological and emotional disturbances leading to violent or addictive behaviors. Time to hit the hay early! But in the meantime, let's sleep soundly, confident that science will gradually provide us with all the explanations!