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Sleep Calculator

Calculate optimal bedtimes or wake-up times based on 90-minute sleep cycles with fall-asleep offset.

Sleep cycle planner

Updates as you type
Plan
What do you want to plan? ?
From which direction? ?
Target wake time
Wake up at 07:00
About you (optional)
Age group ?
Time to fall asleep ? 15 min
min
0153045

How the cycle math works

Tbed = Twake − ( N × 90 min ) − F
Tbed
Time to start winding down for bed
Twake
Target wake time
N
Number of 90-minute sleep cycles (4, 5, or 6)
F
Fall-asleep delay — your sleep onset latency
Worked example — your numbers
  1. Target wake = —
  2. 5 cycles × 90 min = 450 min (7.5h)
  3. Fall-asleep delay F = —
  4. Total offset = —
  5. Recommended bedtime = —

A healthy adult cycles through four stages roughly every 90 minutes: light sleep (NREM 1 & 2), deep slow-wave sleep (NREM 3), and REM. Waking at the end of a cycle — when sleep is light — leaves you refreshed; waking mid-deep-sleep causes that groggy feeling called sleep inertia.

Early cycles are dominated by deep NREM 3 (physical recovery). Later cycles shift more time into REM, which consolidates memory and emotional processing — which is why cutting sleep short by even one cycle disproportionately costs you REM.

Examples

How It Works

The sleep calculator optimizes your sleep schedule using the science of sleep cycles.

Sleep cycles: A complete sleep cycle lasts approximately 90 minutes and progresses through light sleep, deep sleep, and REM sleep. Waking between cycles (rather than mid-cycle) helps you feel more refreshed.

Fall-asleep time: The calculator accounts for an average 15-minute fall-asleep period. This means if you need to be asleep by 11:00 PM, you should be in bed by 10:45 PM.

Two modes: In "wake up at" mode, the calculator works backward from your alarm time to find optimal bedtimes (3-6 cycles). In "go to bed at" mode, it works forward to find optimal wake times. The 5-cycle option (7.5 hours) is highlighted as the typical recommendation for adults.

Tips & Best Practices

The 5-cycle option (7.5 hours) works best for most adults. Try it for a week before adjusting.
If you consistently need more than 15 minutes to fall asleep, mentally add that extra time.
Waking at the end of a cycle (during light sleep) makes you feel more alert than waking mid-cycle.
Teenagers typically need 8-10 hours (5-6 cycles), while older adults may do well with 4-5 cycles.

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Frequently Asked Questions

Why 90-minute cycles?

Research shows the average complete sleep cycle lasts about 90 minutes, progressing through N1 (light), N2 (deeper), N3 (deepest), and REM stages. While individual cycles vary slightly (80-120 minutes), 90 minutes is the most widely accepted average.

Five 90-minute cycles total 7.5 hours of sleep, which falls within the recommended 7-9 hours for adults. This provides enough deep sleep for physical recovery and enough REM sleep for cognitive function.