The cycle from light sleep to REM
A night of sleep moves through repeating cycles of roughly 90 minutes, each passing through non-REM stages and into REM. NREM stage 1 is the brief drowsy transition, stage 2 occupies the largest share of total sleep and features sleep spindles, and stage 3 is slow-wave or deep sleep, the most physically restorative phase. REM sleep, where most vivid dreaming occurs, lengthens across the night while deep sleep dominates the early cycles.
This staged structure matters because different sleep aids tend to influence different parts of the night rather than improving sleep uniformly.
Where common supplements are thought to act
Melatonin acts mainly on the timing of sleep onset rather than the depth of any single stage. Magnesium and glycine are studied for easing the transition into sleep, while some compounds are claimed to affect slow-wave or REM proportions. Many of these stage-specific claims rest on small studies, and prescription sedatives in particular can increase total sleep time while altering normal stage architecture.
This is general educational information, not medical advice. Concerns about sleep stages or persistent poor sleep are best assessed by a qualified clinician.