Your brain sleeps in cycles of about 90 minutes. Waking up mid-cycle is what leaves you groggy — this calculator finds bed or wake times that land right at the edge of a cycle.
One sleep cycle — light sleep, deep sleep, then REM — lasts about 90 minutes on average. Waking right as a cycle wraps up, instead of in the middle of deep sleep, leaves you far more refreshed, even with the same total hours slept.
This calculator also adds about 15 minutes, the average time it takes to actually fall asleep, so the result is more accurate than just dividing by 90.
Sleeping and waking at roughly the same time every day — weekends included — trains your circadian rhythm, making it easier to fall asleep quickly and wake up without hitting snooze five times.
Bright light, including from screens, delays melatonin production.
Its effects can linger in your system for 6–8 hours.
Around 18–20°C (64–68°F) is generally most comfortable for sleep.
This is an estimate based on an average cycle length (90 minutes) and an average time to fall asleep (15 minutes). Everyone's cycle length varies by a few minutes, so treat the result as a solid starting point rather than an exact number.
Most adults feel most refreshed with 5–6 full cycles, which works out to 7.5–9 hours of sleep. That's why those two options are flagged as recommended.
No. Every calculation runs directly in your browser — no times are ever sent to or stored on any server.