Sleep isn't just about hours—it's about cycles. Each cycle lasts around 90 minutes and includes light, deep, and REM sleep. Waking at the end of a cycle feels more natural and leaves you more refreshed.
Sleep researchers recommend planning bedtimes backward from your wake time using 90-minute blocks, allowing you to wake up at the optimal part of a cycle.
Interrupting a cycle—say, waking in deep sleep—can leave you groggy and less focused. Apps and wearables now help people time their sleep more intelligently.
If you can't get a full night's sleep, even 3 or 4 full cycles can leave you feeling significantly better than 8 hours of interrupted rest.