What is non-REM sleep?
Short Answer
Non-REM sleep includes stages N1, N2, and N3 (deep sleep) and makes up most of the night. It features slower brain rhythms and supports physical recovery and memory consolidation.
Why This Matters
Non-REM matters because slow waves and spindles help stabilize learning, regulate the nervous system, and reduce arousal, which leads to restorative sleep. Deep non-REM also raises the threshold for waking, resulting in that “dead asleep” feeling. When non-REM is disrupted, people often wake unrefreshed even if total sleep time looks adequate.
Where This Changes
Non-REM tends to dominate early in the night and decreases with age. Alcohol and sleep deprivation can change non-REM depth and timing, and disorders like apnea can fragment it without you fully noticing.