Sleep Calculator
Calculate when to go to bed, when to wake up, or how many hours of sleep fit between two times. Set your sleep target and include how long you usually take to fall asleep.
Aim to get into bed by
10:45 PM
previous day
- Estimated sleep
- 8 hr
- Time in bed
- 8 hr 15 min
- Wake up at
- 7:00 AM
Other sleep targets
Rough cycle comparison
Uses 90 minutes only as a planning comparison.
How to use the Sleep Calculator
Choose Wake up at, Go to bed at, Sleep now, or Hours slept.
Enter the relevant clock time, choose a sleep target when needed, and include your estimated time to fall asleep.
The calculator separates estimated sleep from total time in bed. Its optional 90-minute comparisons are shown separately rather than replacing the sleep duration you choose.
What time should I go to bed?
If you know when you need to wake up, choose Wake up at. The calculator works backward from that time.
It subtracts both the amount of sleep you want and the time you expect to spend falling asleep.
Here, S is your target amount of sleep and L is your estimated sleep latency.
For example, an 8-hour sleep target with 15 minutes to fall asleep requires 8 hours 15 minutes in bed. Waking at 7:00 AM therefore gives a bedtime of 10:45 PM.
What time should I wake up?
If you already know when you plan to get into bed, choose Go to bed at. The calculator works forward from that time.
Choose Sleep now when you are going to bed immediately. The browser's local time is captured when you press Calculate rather than continuously changing while you read the result.
How many hours did I sleep?
Choose Hours slept when your bedtime and wake-up time are already known.
The calculator measures the interval across midnight when needed, then subtracts your estimated fall-asleep time.
Time spent trying to fall asleep is counted as time in bed but not as estimated sleep. The calculator cannot know about awakenings that happen after you fall asleep.
Sleep targets and approximate sleep-cycle timing
The main calculator is based on the amount of sleep you choose. Presets are available for 7, 8, and 9 hours, with a custom target up to 14 hours.
Separate 7.5-hour and 9-hour results show what five or six 90-minute intervals would look like.
These are planning comparisons, not a claim that your body follows an exact 90-minute timer. An exact cycle multiple should not be used as a reason to unnecessarily shorten your total sleep.
How much sleep do adults need?
CDC guidance varies by age. These values describe general population guidance rather than a personalised target determined by this calculator.
| Adult age | Sleep per day |
|---|---|
| 18–60 years | 7 or more hours |
| 61–64 years | 7–9 hours |
| 65 years and older | 7–8 hours |
Are sleep cycles really 90 minutes?
Not exactly. NHLBI says the sequence of sleep stages generally starts over every 80 to 100 minutes and usually repeats four to six times per night.
Sleep architecture also changes through the night. Deep sleep tends to be more prominent earlier, while REM periods generally become longer later.
A 90-minute value is therefore useful as a rough comparison, but a clock calculator cannot know the length of your next sleep cycle or identify the stage present when an alarm rings.
Why you can still wake up tired
Sleep inertia is the temporary grogginess and reduced performance that can follow waking. Prior sleep loss, biological time, sleep depth, and individual factors can influence it.
Sleep quality, repeated awakenings, circadian timing, accumulated sleep loss, illness, medication, pain, breathing problems, and sleep disorders can also affect how rested you feel.
A calculated clock time cannot guarantee that you will wake feeling refreshed. Persistent sleep problems or substantial daytime sleepiness are worth discussing with a qualified healthcare professional.
Sleep Calculator examples
Wake up at 7:00 AM
With an 8-hour sleep target and 15 minutes to fall asleep, aim to get into bed at 10:45 PM.
Go to bed at 11:00 PM
With an 8-hour target and a 15-minute fall-asleep estimate, wake-up time is 7:15 AM the next day.
Sleep from 11:00 PM to 7:00 AM
The interval gives 8 hours in bed. With 15 minutes allowed for falling asleep, estimated sleep is 7 hours 45 minutes.
Frequently asked questions
What time should I go to bed?
Choose Wake up at, enter the time you need to wake up, choose how much sleep you want, and enter how long you normally take to fall asleep. The calculator subtracts both values from your wake-up time to estimate when you should get into bed.
What time should I wake up?
Choose Go to bed at, enter your planned bedtime, choose a sleep target, and add your usual fall-asleep time. The calculator adds the fall-asleep estimate and sleep target to produce an estimated wake-up time.
How much sleep will I get?
Choose How much sleep?, enter your bedtime and wake-up time, and add your estimated time to fall asleep. The calculator measures the time in bed across midnight when needed, then subtracts the fall-asleep estimate to show estimated sleep.
If I sleep now, when should I wake up?
Choose Sleep now, select the amount of sleep you want, and enter your fall-asleep estimate. When you calculate, the tool takes a snapshot of your browser's current local time and works forward to an estimated wake-up time.
Why does the calculator ask how long I take to fall asleep?
Getting into bed and falling asleep are not the same event. The calculator keeps sleep latency separate so an eight-hour sleep target with a 15-minute fall-asleep estimate requires eight hours and 15 minutes in bed.
Does this sleep calculator use 90-minute sleep cycles?
Only as an optional comparison. Your chosen sleep duration is the primary calculation. The calculator also shows rough 7.5-hour and 9-hour comparisons corresponding to five and six 90-minute intervals, but it does not assume your real sleep stages follow an exact timer.
Are sleep cycles exactly 90 minutes?
No. NHLBI says the sequence of sleep stages generally starts over every 80 to 100 minutes and usually repeats four to six times per night. Ninety minutes is therefore a convenient planning midpoint, not a measurement of your personal sleep-cycle length.
Is 7.5 hours better than 8 hours because of sleep cycles?
Not necessarily. Real sleep cycles vary between people and across the same night. An eight-hour sleep target should not be shortened simply to reach an exact 90-minute multiple. Adequate total sleep, sleep quality, timing, regularity, and individual needs matter.
How much sleep do adults need?
CDC guidance lists 7 or more hours per day for adults ages 18–60, 7–9 hours for ages 61–64, and 7–8 hours for adults 65 and older. These are general population guidelines rather than personalised prescriptions.
Is 6 hours of sleep enough?
Six hours is below the general CDC recommendation for most adults. The calculator does not treat an exact number of estimated sleep cycles as a reason to reduce total sleep below an appropriate duration.
Why can I still feel tired after 8 hours of sleep?
Clock time alone cannot measure sleep quality. Prior sleep loss, awakenings, circadian timing, sleep inertia, illness, medication, breathing problems, pain, and sleep disorders can all affect how you feel after waking. Persistent sleep problems or substantial daytime sleepiness are worth discussing with a healthcare professional.
Does the calculator handle sleep across midnight?
Yes. Bedtime and wake-up calculations wrap across midnight when necessary. In How much sleep? mode, a wake-up time earlier on the clock than the bedtime is treated as occurring on the following day.
Is this a medical sleep assessment?
No. It performs clock-time planning from the values you enter. It does not measure sleep onset, awakenings, sleep stages, sleep quality, circadian phase, alertness, or a sleep disorder.