Sleep Cycle Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles.
Every calculation runs locally in your browser. Nothing you enter here is sent to a server or stored anywhere.
4.5h sleep
6h sleep
7.5h sleep
9h sleep
For informational and educational purposes only — not medical advice, and not a substitute for consulting a qualified professional about your specific situation. TrueMeasureKit is not liable for decisions made based on these results. See our Terms of Service.
Why cycles, not just hours
Sleep moves through roughly 90-minute cycles of light, deep, and REM sleep. Waking up mid-cycle — during deep sleep especially — tends to produce that groggy, disoriented feeling (sleep inertia), even after a full night. Waking up at the boundary between cycles instead tends to feel noticeably easier, which is why these times are spaced in 90-minute steps rather than round numbers of hours. Most adults do best with 5–6 full cycles (7.5–9 hours); fewer than that is a legitimate option, just short of the generally recommended range.
Frequently asked questions
Why 90-minute cycles specifically?
Sleep progresses through cycles of light, deep, and REM sleep that average roughly 90 minutes each. Waking up near the end of a cycle (rather than in the middle of deep sleep) is associated with feeling more refreshed.
Does this guarantee I'll wake up feeling rested?
No — 90 minutes is an average, and individual cycle length varies somewhat night to night and person to person. It's a useful planning heuristic, not a precise biological guarantee.