We often imagine sleep as a pause: the body stops, the brain shuts down, and we pick up where we left off the next day. Reality is very different. During the night, the body performs precise, organized, and essential work.

A night is not a uniform block. It is divided into cycles of about ninety minutes, repeated four to six times. Each cycle goes through several phases. Deep slow-wave sleep dominates the first half of the night: body temperature drops, muscles relax, tissues repair themselves, and immune defenses strengthen. This is the sleep of physical recovery.

The second half of the night gives more room to REM sleep. This phase rests on an apparent contradiction: the brain is almost as active as during the day, while the body's muscles are completely paralyzed. It is during this phase that most dreams occur, and the brain sorts through the day's information: it consolidates what is worth remembering and lets the rest fade. In other words, we also learn while sleeping.

This balance is fragile. Falling asleep depends on a hormone, melatonin, which the brain only releases in darkness. Bright light in the evening therefore delays its production. Bedtime is pushed back, but wake-up time does not change: it is the last cycles of the night that disappear, precisely those containing the most REM sleep. Memory pays the price.

An adult generally needs seven to nine hours of sleep, teenagers need more. Lost hours accumulate into a debt that the body demands sooner or later, and a single long night is not enough to erase it.