Sleep Cycle Calculator
Work out the best bedtime for a target wake-up time, or the best wake-up time if you're going to sleep now, based on 90-minute sleep cycles. Everything runs in your browser — your times are never sent anywhere.
The time you enter is processed entirely in your browser and is never sent anywhere.
Enter a time and press "Calculate".
Options
| Sleep cycles | Total sleep | Time |
|---|
This calculation assumes a sleep cycle of about 90 minutes and an average of 14 minutes to fall asleep after getting into bed. Actual cycle length and time to fall asleep vary by person and by how tired you are. Use this as a general guide only.
How to use
- Choose "From wake-up time" or "From bedtime".
- Enter the time you want to wake up, or the time you're going to bed.
- Press "Calculate". You'll get 3 to 6 cycle options (4.5 to 9 hours of sleep).
How it works
This is an estimate, not medical advice
Sleep cycle length and the time it takes to fall asleep vary from person to person, and even day to day depending on how tired you are. This result is a general estimate and is not a substitute for medical advice. If you have ongoing sleep concerns, please talk to a doctor.
The 90-minute sleep cycle
Sleep moves through a cycle of deeper (non-REM) and lighter (REM) sleep roughly every 90 minutes. Waking up at the boundary between cycles — during a lighter stage of sleep — tends to feel less groggy than waking up mid-cycle. 90 minutes is an average figure; actual cycle length varies from about 70 to 120 minutes between individuals.
Why 14 minutes to fall asleep
Without accounting for the time between getting into bed and actually falling asleep, a calculated bedtime would drift out of sync with your actual sleep cycles. This calculator uses 14 minutes, the figure used by one of the most widely used sleep cycle calculators. Other sources commonly cite a broader range of 15–20 minutes, so 14 minutes sits close to that range.
The formula
From a wake-up time (for cycle count n):
Candidate bedtime = wake-up time − (n × 90 min + 14 min)
From a bedtime:
Candidate wake-up time = bedtime + 14 min + (n × 90 min)
n ranges from 3 to 6, giving four options corresponding to 4.5, 6, 7.5, and 9 hours of sleep. If a result falls on the previous or next calendar day, that's shown alongside the time.
About your data
All calculations happen entirely in your browser. None of the times you enter are ever sent to a server.
FAQ
Is the time I enter sent anywhere?
No. All calculations happen entirely in your browser, and no data is ever sent to any external server or API.
Is the 90-minute cycle actually accurate?
It's an average, not a precise measurement. Real sleep cycles range from about 70 to 120 minutes and vary by person and by tiredness — if you're sleep-deprived, your first cycle of the night can stretch closer to 120 minutes. Treat these results as a starting point, and pay attention to how you actually feel.
How many hours of sleep should I get?
For adults, roughly 7 to 9 hours per night is a commonly cited general guideline. Of the options this tool shows (3 to 6 cycles, or 4.5 to 9 hours), 5 to 6 cycles (7.5 to 9 hours) land closest to that range. Individual needs still vary.
When should I use "from wake-up time" versus "from bedtime"?
Use "from wake-up time" when you know when you need to get up tomorrow. Use "from bedtime" when you want to know what time to wake up if you go to sleep right now — just enter the current time as your bedtime.
Why does a result say "next day" or "previous day"?
When the total sleep time is long enough, the result can fall on a different calendar day. For example, going to bed at 11pm and sleeping 9 hours (6 cycles) means waking up at 8:14am the next day. This tool only calculates times of day, and flags whenever a result crosses midnight.
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