Calculate your perfect bedtime or wake-up time based on your natural 90-minute sleep cycles. Stop fighting your alarm.
💤 Quick nap calculator
Your brain moves through 90-minute sleep cycles all night. Waking between them is the difference between feeling human and feeling terrible.
Tell us when you need to wake up, or when you plan to go to sleep. Either way works.
We calculate backwards or forwards in 90-minute sleep cycle increments, adding 14 minutes for you to fall asleep.
You get 4 options. Pick the recommended one (5 cycles / 7.5 hours) and set your alarm. Wake at the end of a cycle, not the middle.
Every 90 minutes, your brain cycles through four distinct stages. Understanding them is the key to understanding this calculator.
Each sleep cycle lasts approximately 90 minutes and consists of four stages. You complete 4–6 of these cycles per night, with earlier cycles heavier on deep sleep and later cycles heavier on REM.
NREM Stage 1 & 2 — Light sleep. Your heart rate slows, body temperature drops. Easy to wake from, but important for entering deeper stages.
NREM Stage 3 (Deep Sleep) — The most physically restorative stage. Tissue repair, immune function, and growth hormone release happen here. Hardest stage to wake from.
REM Sleep — Rapid Eye Movement. Your brain is nearly as active as when awake. Dreaming, memory consolidation, and emotional processing occur here. You get more REM in the second half of the night.
If your alarm rips you out of deep NREM sleep (Stage 3), your brain is flooded with adenosine — the chemical that builds up sleep pressure. The result is sleep inertia: a heavy, foggy, disoriented state that can last anywhere from 15 minutes to over an hour, even if you technically slept 8 hours.
The fix is simple: time your alarm to land at the end of a 90-minute cycle, when you're naturally transitioning out of sleep into a lighter stage. That's exactly what this calculator does. No app, no expensive sleep tracker — just maths.
A single sleep cycle lasts approximately 90 minutes for most adults. Each cycle includes light sleep (NREM Stage 1 and 2), deep sleep (NREM Stage 3), and REM sleep. You typically complete 4–6 cycles per night, with the later cycles containing progressively more REM sleep.
To wake up at 7:00 AM feeling fully rested (5 cycles / 7.5 hours), you should go to sleep at approximately 11:16 PM. This accounts for 14 minutes to fall asleep, followed by 5 × 90-minute cycles. For 6 cycles (9 hours), aim for 9:46 PM. For 4 cycles (6 hours minimum), aim for 12:46 AM. Use the calculator above for any wake-up time.
Eight hours is 5.33 sleep cycles — meaning your alarm likely fires during the third of a sixth cycle, right in the middle of deep sleep. This causes sleep inertia: grogginess that can persist for 30–60 minutes. Try sleeping 7.5 hours (5 complete cycles) instead. The total is less but the quality of waking is dramatically better.
Most adults need 5–6 complete cycles per night (7.5–9 hours). 4 cycles (6 hours) is the functional minimum for most people. Below 4 cycles consistently leads to accumulated sleep debt, impaired cognitive performance, weakened immune function, and increased risk of long-term health issues. There is no such thing as a "sleep credit" — you cannot bank extra sleep in advance.
A power nap should last 20 minutes — long enough to enter Stage 2 light sleep (restorative) but short enough to avoid entering deep sleep (which causes grogginess). For a more restorative midday nap, go the full 90 minutes for one complete cycle. Avoid the 30–60 minute danger zone, which is most likely to leave you worse off than before you napped.
6 hours equals 4 complete sleep cycles — the absolute minimum threshold for most adults. Research suggests that consistently sleeping only 6 hours leads to cumulative cognitive impairment equivalent to several nights of total sleep deprivation, even if you stop feeling tired. Short-term, 6 hours is survivable. Long-term, it is not optimal. Aim for 5 cycles (7.5 hours) whenever possible.