Sleep is an active physiological state, far removed from the simple "rest" it is sometimes described as. During the night, the brain consolidates learning, the immune system strengthens, and cells regenerate. Understanding sleep, its cycles, and its disorders allows us to better maintain it and quickly identify when something is wrong.
Sleep, an essential pillar of health
Getting enough sleep allows the body to recover both physically and psychologically. The brain uses this rest to consolidate memory, improve concentration, and optimize learning processes. In babies and children, quality sleep promotes brain maturation and contributes to the development of cognitive and motor skills.
Sleep deprivation has measurable consequences for health. It disrupts the production of leptin, the hormone that regulates appetite, thus promoting increased calorie intake. It also disrupts the secretion of cortisol and growth hormone, which are involved in glucose metabolism, increasing the risk of insulin resistance and type 2 diabetes. On a mental level, insufficient sleep increases the risk of irritability, stress , depression, and hypertension.
Sleep rhythm and its cycles
A night's sleep consists of a succession of 3 to 6 cycles, each lasting between 1 and 2 hours. Within each cycle, three main phases are distinguished: light sleep, deep sleep, and REM sleep.
There is no universally ideal amount of sleep: needs vary according to age, genetics, and individual factors. Early birds are distinguished from night owls, as are long sleepers and short sleepers. On average, a French person sleeps about 7 hours a night, which is 1 hour and 30 minutes less than 50 years ago, according to the National Institute of Sleep and Vigilance (INSV).
Regardless of the total sleep duration, the duration of deep sleep remains relatively constant from person to person. It is the duration of light sleep and REM sleep that varies according to the individual and age . Deep slow-wave sleep is more prevalent until around age 20, then gradually becomes less frequent. REM sleep, longer in the early years of life, shortens from adulthood onward. To understand these phases in detail, our article on the different sleep phases explains the role of each.
The study of sleep: polysomnography
The gold standard for studying sleep is called polysomnography. It records physiological variables during the night: brain activity via electroencephalogram (EEG), muscle activity via electromyogram, eye movements via electrooculogram, and heart and respiratory rates. The examination is painless and risk-free. It allows for the precise identification of sleep disorders and the determination of their cause.
Other tools complement this assessment: functional MRI and PET scans allow for the study of brain structures active during each phase. Keeping a sleep diary, in which bedtimes, wake-up times, and any awakenings are recorded daily, is also useful for identifying patterns of insomnia or disrupted sleep rhythms.
The main sleep disorders
According to the INSV, approximately one-third of the French population suffers from sleep disorders. The causes are numerous: overly heavy meals, excessive caffeine or tobacco use, stress and anxiety , underlying medical conditions, medication, or an unsuitable environment (heat, noise, light).
The most frequent sleep disorders are as follows. Insomnia is characterized by difficulty falling asleep or maintaining continuous sleep, leading to daytime fatigue and mood swings. Beyond three weeks, it is considered chronic. Hypersomnia corresponds to the opposite extreme: excessively long nighttime sleep and significant daytime sleepiness. Narcolepsy, a specific form of hypersomnia, is characterized by frequent and involuntary sleep attacks.
Parasomnias encompass abnormal phenomena occurring during sleep: sleepwalking (involuntary movements during deep sleep), nightmares, and violent movements during REM sleep. Circadian rhythm disorders reflect a dysfunction of the biological clock, with sleep and wake times deviating from social norms. Sleep apnea is a separate disorder, characterized by interruptions in breathing during the night.
How to promote better sleep naturally
In cases of restless sleep or difficulty falling asleep, several natural approaches can be considered before resorting to medication. Sedative plants such as passionflower, linden, valerian, or poppy are known for their calming properties on the nervous system. Melatonin, the natural sleep hormone, can help resynchronize the biological clock when rhythms are disrupted.
Studies published in PubMed suggest that melatonin reduces the time it takes to fall asleep and improves sleep quality in cases of mild to moderate insomnia. These supplements should be taken as part of a healthy lifestyle: a balanced diet, daytime physical activity, and a suitable bedroom environment. For more information, our articles on natural remedies for insomnia and tips for sleeping well detail the most effective strategies.
If the problems persist and become chronic, a medical consultation with a doctor or sleep specialist remains essential.
Frequently Asked Questions about Sleep
How many sleep cycles per night is it normal to have?
A normal night's sleep consists of between 3 and 6 cycles, each lasting an average of 90 minutes. The first cycles of the night are richer in deep sleep, while the last are richer in REM sleep. This is why waking up in the late night disrupts memory consolidation more than waking up in the early night.
What is the difference between light, deep, and REM sleep?
Light sleep is the transitional phase between wakefulness and sleep: the body relaxes and brain activity slows down. Deep sleep is the most physically restorative phase: muscles relax completely and growth hormone secretion is at its peak. REM sleep, characterized by intense brain activity and rapid eye movements, is the phase of dreaming and memory consolidation.
At what point does insomnia become chronic?
Insomnia is considered chronic when it lasts more than three weeks and occurs at least three nights a week. Below this threshold, it is referred to as transient or acute insomnia, often linked to an identifiable stressor. Chronic insomnia requires specific treatment, ideally cognitive behavioral therapy, before resorting to sleeping pills.