Sleep Calculator
Find the best bedtimes or wake times based on 90-minute sleep cycles.
About Sleep Calculator
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
Worked example
With the default values below, Sleep Calculator returns Suggested Times of 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. Change any input above and every figure here is recalculated in your browser.
Inputs used
- I want to know…one of When to go to bed (given my wake-up time), When to wake up (given my bedtime)
- When to go to bed (given my wake-up time)
- Desired Wake-Up Time
- 07:00
- Bedtime (when you get into bed)
- 23:00
Results
- Suggested TimesEach option corresponds to waking at the end of a full sleep cycle.
- 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
- 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
- 1Target wake-up time= 7:00 AM
- 2Sleep latency added= 15 minutes (time to fall asleep)
- 3Each sleep cycle= 90 minutes (≈ 90 min of NREM + REM)
- 4Suggested bedtimes (3–6 cycles, latest to earliest)= 9:45 PM … to … 2:15 AM
- 5Recommended (5 cycles = 7.5 h)= 11:15 PM
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.