Bedtime Calculator (90-Minute Cycles)
Enter the wake-up time in HH:MM and get two times to lie down, counting back 5 and 6 sleep cycles plus 14 minutes to doze off: 07:00 gives 23:16 and 21:46.
Horários sugeridos
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Working backwards from a wake time using 90-minute cycles
Sleep runs in cycles of about 90 minutes, and each one moves from light NREM into deep slow-wave NREM and then REM. Wake up at the end of a cycle and you feel sharp. Get pulled out of deep sleep instead and the grogginess hits hard. The math behind it is bedtime = wake − (cycles × 90 min) − sleep latency, where sleep latency usually sits around 15 minutes. Example: if you want to wake at 6:00, a 22:30 bedtime gives you 5 cycles (7h30m of real sleep), while 00:00 gives you 4 cycles (6h00m). Smart alarms in apps like Sleep Cycle, Pillow and AutoSleep read accelerometer data and nudge the alarm within a 30-minute window ahead of your target.
Applications
People use this to map out international travel and jet lag recovery, to tidy up a daily routine, to chase athletic performance (where the amount of deep sleep drives recovery), and to plan study schedules for vestibulandos and ENEM candidates. In Brazil, ANVISA approved melatonin as a food supplement under RDC 922/2024, usually taken 30–60 minutes before the planned bedtime. The 90-minute idea works for naps too: a 90-minute nap finishes a full cycle and spares you the inertia, whereas a 60-minute one can leave you stuck in deep sleep.
FAQ
Is 6 hours of sleep enough if the cycles line up? Four cycles is the floor for the odd night here and there. But sleeping under 7h as a habit still pushes up cardiovascular and cognitive risk, even when the timing is spot on.
Why do later cycles feel different? Early in the night your cycles are loaded with deep N3 sleep. As the night goes on, the balance tips toward REM, which is when most dreaming happens.
Does the 90-minute figure apply to everyone? It's an average. In practice people fall anywhere from 80 to 120 minutes per cycle, and the length tends to shrink a little as you get older.
Is it better to keep a fixed wake time or fixed bedtime? Sleep researchers put their money on a fixed wake time, weekends included. It's the single strongest anchor for the circadian rhythm.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.