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← Sleep Cycle Calculator

To wake up at 6 AM, go to bed at 10:15 PM Form SC-8

The alarm goes off before the sun bothers. Count back five 90-minute sleep cycles plus 15 minutes to fall asleep and the ledger says: in bed by 10:15 PM. That's when the late news ends and the house goes quiet. The 6 AM person's bedtime isn't early, it's arithmetic.

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

The early-riser page. Alarm before sunrise, bedtime to match.

Input slip · Form SC-8

Pre-filled for the 6:00 AM alarm: the ledger posts the bedtimes below the tear line. Change the time and it re-counts instantly.

Posted entry · Form SC-8-R

IN BED BY 10:15 PMForm SC-8

Tonight. Set the alarm for 6:00 AM.

  • 6 cycles8:45 PM9h 15m in bedthe full monty
  • 5 cycles10:15 PM7h 45m in bed the sweet spot for most people
  • 4 cycles11:45 PM6h 15m in bedthe minimum, don’t make it a habit
What about 3 cycles?

In bed by 1:15 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: 6:00 AM → in bed by 10:15 PM (5 cycles, ~7h 45m in bed)
Full rest: 8:45 PM (6 cycles) · Minimum: 11:45 PM (4 cycles)
Rule of thumb, not medical advice. microtoolsdaily.com/wake-up-at-6am/

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.

How the math works Form SC-8

Ninety-minute cycles, fifteen minutes to fall asleep, counted backward from 6:00 AM. Six cycles back is 8:45 PM, five is 10:15 PM, four is 11:45 PM, each one a point where a cycle ends and the next hasn't begun, which is the whole trick.

The arithmetic is exact; the biology is approximate, because nobody's cycles run a metronomic 90 minutes. Use the stamped time as the target and the other rows as the honest alternatives for the nights that don't go to plan.

Common questions Form SC-8

Is 10:15 PM too early to go to bed?

Only by the clock on the wall. For a 6:00 AM alarm, 10:15 PM buys five full 90-minute cycles plus the 15 minutes it takes to drift off, about seven and a half hours of real sleep. It feels early the first week; by the second, the alarm stops feeling like an ambush.

How many hours of sleep do I get waking at 6 AM?

In bed by 10:15 PM means 7 hours 45 minutes between lying down and the alarm; subtract the ~15 minutes of falling asleep and you get roughly 7 hours 30 minutes asleep. Take the 8:45 PM row and it's 9 hours 15 minutes in bed; take 11:45 PM and it's 6 hours 15.

Should I set my alarm for 6 AM or 6:15 AM?

Keep it at 6:00. The bedtimes on this page are counted backward from exactly 6:00 AM, so every quarter hour you move the alarm moves every bedtime with it. And skip the snooze: 9 minutes of snooze starts a fresh cycle you'll be ripped out of, which is worse than just getting up.

What if I go to bed later than 11:45 PM?

Aim for the next cycle boundary instead of splitting the difference: about 1:15 AM gets you three full cycles before 6:00. It's an emergency minimum, not a plan: one late night is a story, five is a sleep debt with interest.

Why do I feel groggy even after 8 hours?

Probably a mid-cycle wake-up. Eight hours in bed doesn't guarantee the alarm landed between cycles; if it rang 40 minutes into a fresh 90-minute cycle, you surfaced from deep sleep and the grogginess (sleep inertia) can hang around for half an hour. The bedtimes above are engineered to land the alarm on a boundary.

Also in the ledger Form SC-8