Sleep Calculator
Find bedtimes or wake times that fall at the end of a 90-minute sleep cycle.
Learn how it works: The Best Time to Go to Sleep: How Sleep Cycles WorkEach suggestion ends a full 90-minute cycle and allows about 15 minutes to fall asleep.
To wake refreshed at your chosen time, fall asleep at one of these times. Cycle length varies from person to person, so use these as a starting point and adjust to how you feel.
How to use the sleep calculator
Pick the mode that matches your question. Set a wake-up time to get suggested bedtimes, or set a bedtime to get suggested wake times.
- Choose whether you are starting from a wake time or a bedtime.
- Enter the time, or tap the current-time shortcut for going to bed now.
- Read the list of suggestions. Each one ends a full sleep cycle.
- Favor the options marked ideal, which give five or six cycles of sleep.
How sleep cycles work
Sleep is not one flat state. It moves through stages, from light sleep into deep sleep and then into REM, the dreaming stage. One full loop takes roughly 90 minutes, and a normal night repeats it several times.
Within each cycle, the deep stages dominate early in the night and REM stretches longer toward morning. That is why the second half of your sleep feels lighter and more dream-filled. The calculator counts in 90-minute blocks so each suggested time lands at the gentle end of a loop.
Why waking between cycles helps
If an alarm pulls you out of deep sleep, you can wake groggy and slow, a feeling known as sleep inertia. Waking at the end of a cycle, when sleep is naturally lighter, tends to feel smoother and more alert.
The 90-minute figure is an average, not a fixed clock. Real cycles run shorter or longer from person to person and even night to night. Treat the suggestions as a helpful target, then nudge the time a little based on how you actually feel when you wake.
How much sleep you need by age
Sleep needs shift across life. These are common guideline ranges for a full night:
- Teenagers, 14 to 17 years: about 8 to 10 hours.
- Young adults and adults, 18 to 64: about 7 to 9 hours.
- Older adults, 65 and up: about 7 to 8 hours.
Within those bands, the right amount is the one that leaves you rested and steady through the day. Quality matters as much as the count of hours, so a dark, cool, quiet room helps every age.
Frequently asked questions
- How long is one sleep cycle?
- About 90 minutes on average, though it ranges from roughly 70 to 120 minutes and changes through the night. The calculator uses 90 minutes as a practical middle figure for spacing the suggested times.
- Why does the calculator add 15 minutes?
- Most people do not fall asleep the instant they lie down. The tool allows about 15 minutes to drift off, so the suggested bedtimes give your cycles room to start. Adjust it in your head if you usually fall asleep faster or slower.
- How many hours of sleep do I need?
- Most adults do well on seven to nine hours, which is five to six full cycles. Teenagers often need more, and older adults a little less. The best amount is the one that leaves you alert without an alarm dragging you awake.
- Is it bad to wake in the middle of a cycle?
- Waking mid-cycle, especially out of deep sleep, can leave you groggy for a while. It is not harmful, just uncomfortable. Aiming for the end of a cycle is meant to make the moment of waking feel easier.
- Are these times exact?
- No. They are guides built on an average cycle length. Your own cycles vary, so use the suggestions as a starting point and fine-tune by noticing when you wake feeling most refreshed.