- On average, an adult should sleep for 7.5 hours, or five cycles of 1.5 hours. With an average of 6h45 per night, according to Public Health France, the French are today a little far from the mark.
- A “good night”, according to the National Institute of Health and Medical Research, is between 4 and 6 successive cycles of 90 minutes on average. Each cycle is itself made up of three distinct phases: light slow-wave sleep, deep slow-wave sleep and REM sleep.
A good night’s sleep doesn’t have to mean linear, uninterrupted sleep. Moreover, seven or eight hours of sleep in one go, that does not even exist!
“We wake up more than a hundred times a night, and this during perfectly normal sleep,” says Cela Kjærby, co-author of a study conducted by Danish scientists and recently published in the scientific journal Nature.
We are not talking here about insomnia or specific sleep disorders, which require possible medical follow-up, but about awakenings or micro-awakenings, common to all mammals.
To reach these conclusions, researchers from the University of Copenhagen (Denmark) studied the sleep of mice using sensors and microscopic optical fibers. They have thus observed that the brain of rodents “wakes up” more than 100 times a night, but in such a short time that the sleeping ones do not realize it and go back to sleep immediately.
It’s the same thing with human beings: the sleeper doesn’t wake up, strictly speaking, every time – that’s why he doesn’t remember – but his brain activity recovers. well on the way. “You could say that short awakenings reset the brain so it’s ready to store memory when you fall back to sleep”explains Maiken Nedergaard, who led the research.
The cause of this hundred nocturnal awakenings? Noradrenaline, a stress hormone and a neurotransmitter related to adrenaline, explain the researchers. When we sleep, the level of norepinephrine in the blood rises and falls every 30 seconds, following a wave pattern. If it is high, it means that you are briefly awake, if it is low, that you are asleep. It is the fluctuation of norepinephrine levels that makes micro-awakenings possible.
In parallel, the study revealed that the more the level of norepinephrine varies during sleep, the better the memory would be afterwards. “The mice [ayant les plus hauts niveaux de variation de noradrénaline] have developed a ‘super memory’. They had less trouble remembering things they had learned the night before. This suggests that norepinephrine dynamics reinforce sleep processes that affect our memory.souligne Celia Kjærby.
As a reminder, these micro-awakenings are not a sleep disorder in themselves: they only become one if you do not go back to sleep afterwards. In case of chronic fatigue, repeated drowsiness or persistent difficulty concentrating, a medical consultation is recommended. In France, more than one in three adults say they suffer from at least one sleep disorder (insomnia, apnea, etc.).