Sleep cycles and dream phases

In short: Sleep proceeds in recurring cycles of light sleep, deep sleep, and REM sleep, during which the most vivid dreams occur. A complete cycle lasts about 90 minutes and repeats several times per night.

In more detail: The composition of sleep phases changes over the course of the night, with more deep sleep early on and more REM sleep later.

In detail

• Light sleep: The transition from wakefulness to sleep, during which the body begins to relax.
• Deep sleep: The most important phase for physical recovery, cell repair, and strengthening the immune system.
• REM sleep: Rapid Eye Movement characterises this phase, during which the most vivid dreams occur and the brain is very active.
• Muscle atonia: During REM sleep, skeletal muscles are largely paralysed to prevent acting out dreams.
• Memory consolidation: Both deep sleep and REM sleep play important roles in solidifying memories.
• Sleep cycle disruptions: Frequent waking or interrupted sleep prevent fully passing through important phases.
• Age effects: With increasing age, the proportion of deep sleep often decreases, explaining lighter, more easily disturbed sleep (schlafhygiene-besser-schlafen, gehirn-aufbau-und-neuroplastizitaet).