Is sleep a matter of surviving?

We often realise how important sleep is when we don’t sleep, or sleep too little: we feel irritated, we have difficulty concentrating, our memory fails us and our appetite is disturbed. So sleep plays a key role in our overall well-being during the day.

But what are its known functions today ? Does its role go beyond our day-to-day well-being?

One certainty

As far as sleep is concerned, if you did the test of not sleeping at all for a long time (a very long time, in fact, and you probably couldn’t do it) on the pretext that we still don’t really know what it’s for, you wouldn’t have the opportunity to regret it… You’d be dead !

In fact, failing to demonstrate the various functions of sleep, the pioneers of sleep research carried out a test (not on themselves, of course). The premise is simple: to find out what an action is for, all you have to do is prevent it from happening and observe the consequences. Among other observations, researchers1 in the United States were able to show that after an average of 21 days of total sleep deprivation, rats subjected to this treatment died, even though the autopsy revealed no health problems. They then went further and deprived rats of REM sleep or deep sleep. And each time, the rats died under the same conditions, albeit at different times. So we can say that, yes, sleep is essential, one thing is certain: we cannot live without sleep, nor can we live without either REM sleep or deep sleep.

What we know about sleep functions

Beyond the extreme consequences of sleep deprivation, various scientific and empirical studies allow us to list some of the more precise functions of sleep.

In the 1960s, a young man set himself the task of breaking the record for staying awake2. A researcher at the time came along to conduct a scientific experiment. He found that sleep deprivation had a definite impact on cognitive and behavioural functions, but with no consequences once he had finally slept. Above all, he was able to conclude that all the body’s other functions functioned more or less normally. So only the brain and nervous system would really be affected.

But since then, many other functions of sleep have been discovered:

• Sleep ‘restores’ the brain

Contrary to other organs, it is only during sleep that the brain can recover. The space between the nerve cells represents 14% of the brain’s volume. Well, during sleep, this volume increases to 23%, allowing the cerebrospinal fluid (the ‘juice’ of the brain) to circulate more easily, taking with it the toxins accumulated during wakefulness.

• Sleep helps to strengthen the memory

Memory is strengthened when we memorise what we have learnt so that we can play it back later. This consolidation is most effective during sleep, even if it’s only for a short time. This process is not simply a matter of adding mortar to breeze blocks, it’s a real job of analysing and organising the information to integrate it with the memories already recorded. And while all the phases are important for memorisation, the first sorting out of what needs to be retained and what needs to be forgotten takes place during deep sleep.

• Sleep could help secrete growth hormone

Growth hormone is secreted mainly during deep sleep in the first part of the night. It is good for growth, but also for cell regeneration in adults.

• Sleep could help boost immune response

There’s a reason why we sleep more when we’re ill. German researchers demonstrated this in 2011 by having half of the people who had just been vaccinated against hepatitis A sleep through the night, compared with half who had been able to sleep normally. After a year, antibody levels in the first group remained lower than those in the second.

• Sleep may help maintain weight

When we sleep well, our secretion of ghrelin, a hormone that makes us feel hungry, decreases and, conversely, we produce more leptin, the satiety hormone. Clearly, when we sleep well we feel less need to eat and we’re fuller more quickly, so we eat just what we need, not more. Obviously, gluttony is another matter!

This overview of the functions of sleep is just a tiny fraction of what you could learn about sleep. To find out more about this mystery, we recommend reading the excellent book “Je rêve de dormir” (I dream of sleeping) by Dr José Haba-Rubio and Dr Raphaël Heinzer, published by Favre, from which most of the information has been taken.

1.  ^{https://jamanetwork.com/journals/jamaneurology/article-abstract/565718?redirect=true}
2. ^{Lange T, Born J (2011) The immune recovery function of sleep – tracked by neutrophil counts. Brain Behav Immun 25:14-15}