What are sleep cycles?

Sleep is divided into cycles that are about 90 minutes long, consisting of different stages including REM (rapid eye movement) sleep and non-REM sleep. Non-REM sleep has four stages, from light sleep to deep sleep. As the night progresses, the amount of REM sleep increases in each cycle, which has important implications for learning and memory processing 1.

These sleep cycles reflect a biological rhythm that includes deep non-REM sleep, dominated by slow brainwaves and bodily restoration, and REM sleep, where we experience most of our dreaming 2. Throughout the night, there's a dynamic balance between non-REM and REM sleep, with deep non-REM sleep occurring mostly in the first half and more REM sleep in the second half 2.

The architecture of our sleep is influenced by circadian rhythms and sleep pressure. The circadian rhythm is a 24-hour internal clock regulating wakefulness and sleepiness in relation to external light cues. Sleep pressure builds up the longer we're awake due to accumulating adenosine, a chemical that promotes sleepiness 3 4. Circadian rhythms synchronize with other cells in the body, influencing temperature regulation and overall well-being, and they are aligned with the Earth's 24-hour rotation and light-dark cycle 5.