What is a sleep cycle?
A sleep cycle lasts approximately 90 minutes and consists of several stages: light sleep (N1, N2), deep sleep (N3/slow-wave sleep), and REM sleep. Your brain cycles through these stages multiple times per night. Waking up between cycles — rather than in the middle of deep sleep — is what makes you feel refreshed.
Why do I feel groggy even after 8 hours of sleep?
This is called sleep inertia — the grogginess you feel when you wake up during deep sleep. If your alarm goes off in the middle of a sleep cycle (typically N3/deep sleep), you'll feel far worse than if you'd woken up 15 minutes earlier at the natural end of a cycle. This calculator helps you time your sleep to avoid this.
How many sleep cycles do I need?
Most adults need 5–6 complete sleep cycles per night, which equates to 7.5–9 hours of sleep. 4 cycles (6 hours) is the minimum for most people to function, but consistently getting fewer than 5 cycles is associated with health risks. The calculator highlights 5 and 6 cycles as the ideal targets.
Why does the calculator add 14 minutes?
On average, it takes about 14 minutes for a healthy adult to fall asleep after getting into bed. This is called sleep latency. The calculator accounts for this so that the bedtimes shown are when you should get into bed, not when you need to be asleep.