Studies in 2025 confirmed that people who sleep in multiples of 90-minute cycles perform better on memory and reaction tests.
Sleep labs also found better mood stability and heart rate variability in participants who structured their sleep this way.
Rather than sleeping 8 hours and waking in the middle of REM sleep, participants who slept for 6 or 7.5 hours—completing 4–5 full cycles—woke up more refreshed.
Experts now recommend adjusting alarms to avoid interrupting cycles, even if it means waking a little earlier.