Sleep is an active neurophysiological state that allows the body and brain to recover, consolidate learning, and strengthen immune defenses. Regular adult sleep of 7 to 9 hours is associated with a reduced risk of cardiovascular diseases, type 2 diabetes, obesity, and mood disorders.
Biological Mechanisms of Sleep
Sleep results from the interaction between two major systems: the circadian rhythm (internal clock of about 24h) and the homeostatic pressure of sleep (the need to sleep accumulating throughout wakefulness). The suprachiasmatic nucleus of the hypothalamus synchronizes this rhythm thanks to light, regulating sleep onset, body temperature, and hormonal secretion.
- Melatonin, secreted at night, facilitates sleep onset and circadian synchronization.
- Cortisol, high in the morning, supports wakefulness but, when chronically high, increases cardiometabolic risk.
- Serotonin participates in regulating the sleep-wake cycle and mood.
- GABA, major inhibitory neurotransmitter, promotes sleep initiation and maintenance.
Sleep Cycles and Phases
A typical night includes 4 to 6 cycles of 90 minutes alternating non-REM sleep (NREM: N1, N2, N3) and REM sleep. During the night, the portion of deep sleep progressively decreases while REM sleep episodes lengthen, allowing for cumulative physical and mental recovery.
Correspond to light sleep, marked by cardiovascular slowing, sleep spindles, and K-complexes, representing the majority of total sleep time.
2️⃣ N3 Sleep (Deep)Slow-wave deep sleep is the most restorative phase physically, associated with growth hormone release, tissue repair, and strengthening of certain immune functions.
3️⃣ REM SleepCharacterized by rapid brain activity, vivid dreams, and muscle atonia, playing a major role in memory consolidation, emotional regulation, and neural circuit adjustment.
Sleep and Immune System
Sleep and the immune system constantly communicate via cytokines, hormones, and nerve signals. Certain pro-inflammatory cytokines, like IL-1β and TNF-α, promote NREM sleep, which seems to help the body mobilize an effective response against infections.
A sleep deficit leads to low-grade inflammation and alteration of several immune parameters:
- Chronic sleep deprivation decreases the activity of certain lymphocytes.
- It disrupts antibody production.
- It reduces the efficacy of vaccine response.
These immune disturbances contribute to increased susceptibility to respiratory infections and inflammatory or metabolic diseases.
Benefits for the Body
Sufficient sleep plays a major protective role for the heart, metabolism, and longevity.
- Studies associate short or fragmented sleep with an increased risk of hypertension, coronary heart disease, stroke, and cardiovascular mortality.
- Regulation of insulin, leptin, and ghrelin during the night helps stabilize blood sugar, hunger, and body weight, reducing the risk of type 2 diabetes and obesity.
- Deep sleep promotes muscle and tissue repair, as well as a nocturnal drop in blood pressure, which protects vessels in the long term.
- Good sleep also improves recovery after exercise, pain tolerance, and overall physical performance.
Benefits for the Brain and Mental Health
The brain benefits from sleep to consolidate memories, reorganize neural circuits, and "clean" metabolic waste via the glymphatic system. The combination of slow waves in deep sleep and REM phases participates in learning integration, creativity, and emotional adaptation.
Chronic lack of sleep strongly weakens mental health:
- Sleep deprivation increases the risk of depression, anxiety, irritability, burnout, and attention disorders.
- Long-term, poor quality sleep is associated with faster cognitive decline and an increased risk of neurodegenerative disorders.
