Sleep Calculator
Sleep consists of 90-minute cycles. Waking at the end of a complete cycle leaves you feeling refreshed; waking mid-cycle causes grogginess. Use this calculator to find the ideal bedtime (given a wake-up time) or the ideal alarm times (given a bedtime) to complete full sleep cycles.
Frequently Asked Questions
Why does sleep in 90-minute cycles matter?+
A full sleep cycle moves through light sleep, deep (slow-wave) sleep, and REM sleep. Waking at the end of a cycle — when sleep is lightest — feels natural. Waking mid-cycle (especially during deep sleep) causes sleep inertia: that groggy, disoriented feeling that can last for hours.
How many sleep cycles do I need?+
Most adults need 5–6 complete cycles (7.5–9 hours). 4 cycles (6 hours) is the minimum for functional cognitive performance but accumulates sleep debt over time. Consistently sleeping fewer than 5 cycles is associated with impaired memory, mood, and immune function.
What time should I stop using screens before bed?+
Blue light from screens suppresses melatonin production. Research suggests avoiding screens 1–2 hours before your target bedtime, or using night-shift / red-filter modes to reduce blue-light emission from around 9 PM.