Sleep Calculator
Tell it when you need to wake up and it lists three bedtimes that allow 6, 5 or 4 cycles of 90 minutes after a 15-minute wind-down. Or press a button to see when to set an alarm if you go to bed now. It is arithmetic on an average, not a medical measurement. Free, no sign-up.
Mode
Suggested Bedtimes
9:45 PM
9h of sleep
11:15 PM
7.5h of sleep
12:45 AM
6h of sleep
Each bedtime allows ~15 minutes to fall asleep, then 6, 5, or 4 complete 90-minute cycles before your alarm.
How the Math Works
bedtime = wake time − 15 min buffer − (cycles × 90 min)
A full sleep cycle averages about 90 minutes, but that is only an average — individual cycles range from roughly 70 to 120 minutes and shift over the course of the night. Treat these times as a starting point, not a precise prescription. This tool is not medical advice; if you have ongoing sleep problems, talk to a doctor.
How to Use the Sleep Calculator
- 1. Pick a mode. I want to wake up at… gives bedtimes. If I go to bed now… gives wake-up times.
- 2. Enter the time or press the button. For the first mode set your wake-up time. For the second, press Calculate Wake-Up Times, which reads your device's clock; press Recalculate From Now later to refresh it.
- 3. Read the three options. Each card shows a clock time and the amount of sleep it allows: 9 hours, 7.5 hours or 6 hours.
- 4. Choose by how much sleep you need, not by which time looks tidiest. See the guidance below.
The Arithmetic
Bedtime = wake-up time − 15 minutes − (cycles × 90 minutes). The wake-up times in the other mode run the formula forwards: bedtime + 15 minutes + cycles × 90 minutes. Results wrap past midnight, and times are shown on a 12-hour clock.
Example: waking at 7:00 AM
6 cycles
7:00 AM − 15 min − 9 h = 9:45 PM, for 9 hours of sleep
5 cycles
7:00 AM − 15 min − 7.5 h = 11:15 PM, for 7.5 hours of sleep
4 cycles
7:00 AM − 15 min − 6 h = 12:45 AM, for 6 hours of sleep
Example: going to bed at 11:00 PM
Asleep by
11:15 PM
4 cycles
5:15 AM, after 6 hours of sleep
5 cycles
6:45 AM, after 7.5 hours
6 cycles
8:15 AM, after 9 hours
How Much Sleep Do You Need?
The US Centers for Disease Control and Prevention recommends at least 7 hours a night for adults aged 18 to 60, 7 to 9 hours for ages 61 to 64, and 7 to 8 hours from 65. Teenagers aged 13 to 17 are advised 8 to 10 hours. On those figures the 6-hour option is a short night for nearly everyone, and the 7.5-hour option suits most adults. The CDC also recommends keeping the same bedtime and wake-up time every day.
What the 90-Minute Cycle Is, and Is Not
Sleep alternates between non-REM sleep, which has three stages ending in deep slow-wave sleep, and REM sleep. The US National Heart, Lung, and Blood Institute describes the cycle as starting over every 80 to 100 minutes, with most nights containing four to six cycles. Deep sleep dominates early in the night and REM later. So 90 minutes is a sensible average, but yours may be shorter or longer, it changes through the night, and brief awakenings occur between cycles.
The reasoning behind cycle-based wake-up times is sleep inertia, the grogginess after waking. A 2000 review by Tassi and Muzet found that the stage you wake from is one of the most important factors: waking from slow-wave sleep produces more inertia than waking from light stages 1 or 2, with REM in between. That gives the method a rationale, but it does not guarantee that a time 90 minutes from your bedtime lands in light sleep. Treat the times as a starting point, then adjust by a cycle or by a few minutes for a week and see what works.
Limits to Keep in Mind
- The 15 minutes is an assumption. If you usually fall asleep faster or slower, shift each time by the difference.
- It cannot see your sleep. There is no sensor, only a formula, so it cannot tell when you are in deep or light sleep.
- Consistency matters more than precision. A regular schedule beats a perfect-looking time that changes every night.
- It is not medical advice. If you often cannot sleep, or you feel unrefreshed after enough hours, speak to a doctor.
Which Tool Do You Need?
| If you want… | Use |
|---|---|
| A bedtime or wake-up time worked out from sleep cycles | This Sleep Calculator |
| Study sessions and breaks spread across the day | Study Schedule Generator |
| Timed work and rest periods while you study | Pomodoro Timer |
| Steady sound to mask noise while you wind down | White Noise Generator |
| A daily check-off for a sleep routine | Habit Tracker |
Frequently Asked Questions
- How does the sleep calculator work?
- It subtracts 15 minutes for falling asleep and then 6, 5 or 4 blocks of 90 minutes from your wake-up time to give bedtimes, or adds the same amounts to your bedtime to give wake-up times. All arithmetic runs in your browser.
- Why 90 minutes?
- It is a round-number average. The National Heart, Lung, and Blood Institute puts the cycle at 80 to 100 minutes, and the length differs between people and across the night. The calculator cannot know where your own cycle falls in that range.
- How many hours of sleep do I need?
- The CDC recommends 7 or more hours for ages 18 to 60, 7 to 9 for ages 61 to 64, 7 to 8 from 65, and 8 to 10 for teenagers aged 13 to 17. Individuals vary, so judge by how you feel in the day as well.
- Does waking at the end of a cycle really feel better?
- There is a basis for it: waking from deep slow-wave sleep causes more grogginess than waking from light sleep. But cycle length varies, and the end of a cycle is not always light sleep, so the effect from an alarm timed this way is not guaranteed. Getting enough total sleep and keeping a regular schedule matter more.
- Why is there a 15-minute buffer?
- Cycles count from when you fall asleep, not when you get into bed. The calculator assumes about 15 minutes to drift off. If you fall asleep quickly or slowly, adjust each time by the difference.
- Is this medical advice?
- No. It is a calculator built on an average, and it cannot diagnose or treat any sleep problem. If you struggle to sleep or feel exhausted despite enough hours, talk to a doctor or sleep specialist.