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← Sleep Cycle Calculator

Sleep cycle calculator for shift workers Form SC-8

Your 7 AM is everyone else's 7 PM, and the 90-minute math doesn't care what the clock says. Coming off a night shift with a 3 PM wake-up target? Five cycles back puts you in bed by 7:15 AM. Anchor sleep, blackout curtains, and a bedtime your rotating schedule can actually keep.

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

The night-shift page. The math works in daylight too.

Input slip · Form SC-8

Pre-filled for a 3:00 PM wake-up after night shifts: bedtimes posted below the tear line. Change the time and it re-counts instantly.

Posted entry · Form SC-8-R

IN BED BY 7:15 AMForm SC-8

Tonight. Set the alarm for 3:00 PM.

  • 6 cycles5:45 AM9h 15m in bedthe full monty
  • 5 cycles7:15 AM7h 45m in bed the sweet spot for most people
  • 4 cycles8:45 AM6h 15m in bedthe minimum, don’t make it a habit
What about 3 cycles?

In bed by 10:15 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: 3:00 PM → in bed by 7:15 AM (5 cycles, ~7h 45m in bed)
Full rest: 5:45 AM (6 cycles) · Minimum: 8:45 AM (4 cycles)
Rule of thumb, not medical advice. microtoolsdaily.com/shift-worker/

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

A sleep cycle is about 90 minutes whether the sun is up or not: light sleep down into deep sleep, up through REM, repeat. From a 3:00 PM wake target, five cycles plus 15 minutes to fall asleep means lights out at 7:15 AM; six cycles reaches 5:45 AM, four reaches 8:45 AM.

The 90-minute math doesn't care what the clock says. Your blackout curtains do the convincing, the arithmetic just picks the landing times. Individual cycles vary, so treat these as the best-aimed targets for a sleep window the daylight wants to steal.

Common questions Form SC-8

Does the 90-minute cycle math work for daytime sleep?

The cycles don't check the clock. The ~90-minute rhythm of light, deep, and REM sleep runs on your body's internal schedule, not the sun's. What changes with daytime sleep is how easily you're woken, not the architecture of the sleep itself. So yes: five cycles is five cycles at 7 AM or 7 PM.

How do I protect a 7:15 AM bedtime from daylight and noise?

Make the bedroom lie about the time: blackout curtains or a sleep mask for the light, earplugs or white noise for the daytime soundscape, and the phone on do-not-disturb so the waking world can't reach you. Cool the room down too: your body expects falling temperatures with sleep, and daylight argues otherwise.

What's anchor sleep and why does it matter for rotating shifts?

Anchor sleep is a fixed core block, usually at least 4 hours, that you keep at the same clock time every day, even across rotations, with the rest of your sleep flexing around it. It gives your circadian rhythm one stable appointment to plan around when everything else moves. Pair it with the cycle math: make the anchor a whole number of cycles.

Can naps replace a missed cycle?

A 20-minute nap stays in light sleep and takes the edge off without grogginess; a full 90-minute nap completes one cycle cleanly. Anything between tends to wake you from deep sleep feeling worse. Naps are triage for a missed cycle, not a replacement program. The ledger still wants its full entry most days.

Also in the ledger Form SC-8