Sleep Calculator
Find the best bedtimes or wake times based on 90-minute sleep cycles and a 15-minute sleep latency.
Inputs
Enter the time you need to be awake.
Suggested Times
9:45 PM — 9h of sleep (6 cycles) · 9h 11:15 PM — 7h 30m of sleep (5 cycles) · 7.5h 12:45 AM — 6h of sleep (4 cycles) · 6h 2:15 AM — 4h 30m of sleep (3 cycles) · 4.5h
Each option corresponds to waking at the end of a full sleep cycle.
Best Option (5 cycles, 7.5 h)
11:15 PM
Note
Includes 15-minute sleep latency. Recommendations are for healthy adults; individual needs vary.
Step by step
Target wake-up time
= 7:00 AM
Sleep latency added
= 15 minutes (time to fall asleep)
Each sleep cycle
= 90 minutes (≈ 90 min of NREM + REM)
Suggested bedtimes (3–6 cycles, latest to earliest)
= 9:45 PM … to … 2:15 AM
Recommended (5 cycles = 7.5 h)
= 11:15 PM
How it works
Human sleep is not a uniform state — it alternates between NREM (light sleep, deep sleep) and REM (dreaming) in roughly 90-minute cycles. Waking in the middle of a deep-sleep stage (NREM3) causes 'sleep inertia': the groggy, disoriented feeling that can last for up to an hour. Timing your alarm to coincide with the end of a cycle — when sleep is naturally lightest — minimises sleep inertia. We add 15 minutes to account for the average time it takes a healthy adult to fall asleep after getting into bed. The National Sleep Foundation recommends 7–9 hours for adults, which corresponds to 5–6 full 90-minute cycles.
Formula
Bedtime from wake time
Bedtime = Wake time − (n × 90 min) − 15 min sleep latency
- n
- Number of sleep cycles (3–6)
- 90 min
- Duration of one full sleep cycle
- 15 min
- Average sleep-onset latency for healthy adults
Frequently Asked Questions
Do sleep cycles really last exactly 90 minutes?
No — 90 minutes is the population average. Early-night cycles tend to be heavier on NREM deep sleep; late-night cycles are heavier on REM. Individual cycle length can vary from 70 to 120 minutes. Smart sleep trackers that measure movement or heart rate can estimate your personal cycle length more accurately.
How many sleep cycles do I need?
The National Sleep Foundation recommends 7–9 hours for adults (18–64), which is roughly 5–6 full cycles. Most healthy adults function well on 5 cycles (7.5 h). Teenagers typically need 8–10 hours; school-age children need 9–11 hours.
What is sleep inertia?
Sleep inertia is the grogginess and impaired alertness that occurs immediately after waking, especially when woken from deep sleep (NREM3). It can impair reaction time and decision-making for 15–60 minutes. Waking at the end of a light stage minimises it.
What if I can't fall asleep in 15 minutes?
If you consistently take longer than 20–30 minutes to fall asleep, you may have sleep-onset insomnia and sleep hygiene changes (consistent schedule, dark cool room, no screens before bed) could help. For the calculator, you can adjust by going to bed 15–30 minutes earlier than suggested.