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Sleep cycle calculator: what time should you go to bed? Form SC-8

Tell it when the alarm goes off and it counts backward through 90-minute sleep cycles,with 15 minutes to drift off, and stamps the bedtimes that let you wake up between cycles instead of inside one. Or tap the night-owl button and it hands you the wake-up times from right now.

Tool № 8 · Saturday, September 26, 2026 · free, always

Counted in cycles, not hours.

Input slip · Form SC-8

Fill in the slip and the ledger posts your bedtimes below the tear line. Nothing leaves your browser; your alarm time never travels further than this page.

Posted entry · Form SC-8-R

IN BED BY 10:45 PMForm SC-8

Tonight. Set the alarm for 6:30 AM.

  • 6 cycles9:15 PM9h 15m in bedthe full monty
  • 5 cycles10:45 PM7h 45m in bed the sweet spot for most people
  • 4 cycles12:15 AM6h 15m in bedthe minimum, don’t make it a habit
What about 3 cycles?

In bed by 1:45 AM: 4h 45m in bed, roughly four and a half hours of actual sleep. Emergency minimum, not a plan.

My bedtime ledger — Form SC-8
Wake: 6:30 AM → in bed by 10:45 PM (5 cycles, ~7h 45m in bed)
Full rest: 9:15 PM (6 cycles) · Minimum: 12:15 AM (4 cycles)
Rule of thumb, not medical advice. microtoolsdaily.com/tools/sleep-cycle-calculator/

A rule of thumb, not medical advice. This calculator uses average sleep-cycle lengths to estimate good bedtimes. It doesn’t know you, can’t diagnose anything, and doesn’t promise how you’ll feel. If sleep is a real problem, talk to a doctor — not a ledger.

Common schedules, pre-filled Form SC-8

The usual suspects: each one opens the calculator with the schedule already entered:

How the math works Form SC-8

Sleep runs in cycles of about 90 minutes: light sleep, deep sleep, then REM, then back around again. This page adds 15 minutes for the time it takes most people to drift off, then counts whole cycles backward from your alarm: for a 6:00 AM wake-up, five cycles lands at 10:15 PM, six at 8:45 PM, four at 11:45 PM.

Wake between cycles and you surface cleanly; wake mid-cycle and the alarm feels like a personal insult. Ninety minutes is an average, not a law: real cycles run roughly 80 to 120 minutes and shift through the night, so treat the stamped time as the target and the other rows as the honest alternatives.

Common questions Form SC-8

What is a sleep cycle, really?

A sleep cycle is one full pass through the stages of sleep: light sleep, deep sleep, then REM (dream) sleep, lasting about 90 minutes. You run four to six of them a night. Waking at the end of a cycle, during light sleep, feels dramatically easier than being dragged out of deep sleep mid-cycle, which is the entire idea behind this page.

Is 90 minutes exact?

Ninety minutes is the species average, not a personal constant. Real cycles run roughly 80 to 120 minutes and tend to shift through the night, with more deep sleep early and more REM toward morning. The 90-minute rule of thumb is exact enough to pick good bedtimes and honest enough to offer three of them.

Why do you add 15 minutes?

The 15 minutes is the average time it takes to fall asleep after lying down (sleep researchers call it sleep latency). The cycle countdown starts when you drift off, not when your head hits the pillow, so the page pads every bedtime by a quarter hour to land the cycles correctly.

What if I can't fall asleep in 15 minutes?

Then shift the bedtime earlier by your real wind-down time. If you reliably need 30 minutes to drop off, treat the stamped time as lights-out-minus-15 and get in bed 15 minutes sooner. The cycles are counted from sleep onset, so the fix is always on the going-to-bed side, never on the alarm side.

Is 4 cycles (6 hours) enough?

For one rough night, survivable. As a routine, no: most adults need 7 to 9 hours, and a chronic 6-hour habit accrues a sleep debt that caffeine only refinances. The 4-cycle row is the floor of the house, not a room you should live in.

Is this medical advice?

No. This page does arithmetic on averages: 90-minute cycles and a 15-minute drift-off that describe populations, not you specifically. It can't diagnose anything and promises nothing about how you'll feel. If sleep is a real, persistent problem, talk to a doctor — not a ledger.

Also in the ledger Form SC-8