The basic structure of a sleep cycle
Sleep moves through several stages, including light sleep, deep sleep, and REM sleep, cycling roughly every 90 minutes throughout the night — each stage serves genuinely different restorative functions.
Why deep sleep matters particularly for physical recovery
Deep sleep is when the majority of growth hormone release and physical tissue repair occurs, making it particularly important for anyone training regularly, beyond just feeling rested.
Why REM sleep matters for mental and emotional processing
REM sleep is associated with memory consolidation and emotional processing — getting insufficient REM sleep, even with adequate total hours, can affect mood and cognitive function the following day.
Why waking up mid-cycle feels particularly rough
Waking during deep sleep tends to produce a groggier, harder start to the day compared to waking during lighter sleep stages — this is part of why sleep quality, not just duration, meaningfully affects how rested someone feels.