Stax

Sleep Calculator

Find ideal bedtime or wake-up time based on 90-minute sleep cycles.

Go to sleep at…
09:46 PM
Ideal — fully rested
6 cycles
9h sleep
11:16 PM
Good — recommended minimum
5 cycles
7.5h sleep
12:46 AM
Fair — may feel tired
4 cycles
6h sleep
02:16 AM
Poor — use only in emergencies
3 cycles
4.5h sleep

Based on 90-minute sleep cycles and ~14 minutes to fall asleep.

A full night of sleep is typically 5–6 complete cycles (7.5–9 hours).

Why sleep cycles matter more than total hours

Most sleep advice focuses on “get 8 hours.” But sleep quality depends just as much on whenyou wake up within your cycles. Waking mid-cycle — especially during deep N3 sleep — triggers sleep inertia, the heavy grogginess that can last 30–60 minutes even after a full night’s rest.

How to use this calculator

  • I want to wake up at… — Enter your target wake time. The calculator shows you the ideal bedtimes for 3–6 complete cycles.
  • I’m going to sleep at… — Enter when you plan to sleep. The calculator shows when to set your alarm for a smooth wake-up.

Tips for better sleep

  • Keep a consistent sleep and wake schedule — even on weekends.
  • Avoid screens 30–60 minutes before bed; blue light suppresses melatonin.
  • Keep the room cool (16–19°C / 60–67°F) for deeper slow-wave sleep.
  • Avoid caffeine after 2 PM as it has a half-life of about 5–6 hours.

Frequently asked questions

How does the sleep calculator work?
It uses the 90-minute sleep cycle model. Each full cycle takes about 90 minutes and includes light sleep, deep sleep, and REM. Waking at the end of a cycle — rather than in the middle — significantly reduces grogginess. The calculator also adds about 14 minutes for the average time it takes to fall asleep.
How many sleep cycles do I need?
Most adults need 5–6 complete cycles per night, which equals 7.5–9 hours of sleep. Four cycles (6 hours) is the practical minimum for most people. Six cycles (9 hours) is often recommended for teenagers and adults recovering from sleep debt.
What is the 14-minute fall-asleep buffer?
On average, it takes a healthy adult about 10–20 minutes to fall asleep after getting into bed. The calculator uses 14 minutes as a representative average. You can mentally adjust the times if you fall asleep faster or slower.
Why do I feel groggy even after 8 hours of sleep?
Grogginess (sleep inertia) usually means you woke up in the middle of a deep sleep phase rather than at the end of a cycle. Try adjusting your alarm by 15–30 minutes earlier or later to land at a cycle boundary.
What happens during a sleep cycle?
Each cycle progresses through: N1 (light sleep, ~5 min), N2 (stable sleep, ~25 min), N3 (deep/slow-wave sleep, ~30 min), and REM (rapid eye movement, ~30 min). Deep sleep dominates early in the night; REM sleep dominates later cycles.

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