The four stages of sleep

• You fall asleep, sometimes experiencing muscle contractions or jolts.
• You fluctuate between wakefulness and sleep. This period is generally short-lived.
• You are aware of surrounding noises.

• Eye movements stop, heart rate slows and body temperature drops.
• Between 45% and 55% of your night is spent in sleep stage 2. This stage is crucial for your physical and mental health, among other things.

• Brain waves become slower, but increase in amplitude.
• This stage is important especially for physical recovery, strengthening the immune system, regulating appetite and growth.

• You enter the dream phase.
• Your heart rate and breathing quicken, while your blood pressure rises.
• Your eyes move rapidly and irregularly.
• The muscles in your arms and legs become paralyzed.

During the first cycles of the night, deep sleep (stage 3) is predominant, while the duration of REM sleep gradually lengthens. In the event of sleep disruption, the body first seeks to return to deep sleep (stage 3) and REM sleep. Each stage of sleep is important; it’s the regular, uninterrupted series of cycles that is essential for restful sleep.

Sleep also varies throughout life:[3] While we’re growing, slow-wave sleep is deeper, reaching its peak around the age of 20. As we get older, this type of sleep is less frequent, giving way to lighter, slow-wave sleep, which partly explains the increase in sleep disorders with age. At the same time, REM sleep lasts longer in the early years, but decreases in adulthood.

Adults need around eight hours of sleep a night, but this can vary. Some people feel rested after six hours, while others need 10 hours to be fully rested.

The contribution of REM sleep and dreams is complex to characterize, partly due to the difficulty of studying the impact of REM sleep deprivation without disturbing deep sleep.[4] REM sleep is thought to play an important role in reinforcing “declarative” memory, which is expressed in words, as opposed to non-declarative memory, which is linked to innate learning. It is also very important for brain development in newborns. In addition, REM sleep prepares the body for wakefulness. Indeed, REM sleep is always followed by wakefulness, and its importance increases throughout the night.

The other roles of REM sleep and dreaming are more difficult to determine. During REM sleep, the most intense and memorable dreams occur. Their precise function is still the subject of intense research. Some theories, such as that of Freud, suggest that they reflect repressed desires, while others believe they help us memorize, revise or integrate aspects of our experience. On the other hand, some people believe that dreams are the result of a sudden reactivation of consciousness on awakening, or a random activity of the brain with no particular meaning.[3]