Sleep Calculator

How the sleep calculator works

Sleep isn't one long, even block. Through the night, your brain moves through repeating sleep cycles of light sleep, deep sleep and REM (dreaming) sleep. A full cycle takes roughly 90 minutes, and most adults go through four to six a night.

Waking up at the end of a cycle, when you're already in light sleep, usually feels easier than waking up in the middle of deep sleep. That's why 7.5 hours of sleep can sometimes feel better than 8.

The calculator counts backwards (or forwards) in 90-minute steps from the time you give it, and adds about 15 minutes for falling asleep. Under Adjust you can change that, and set your age so the right number of cycles is marked Recommended.

Three ways to use it

• Wake up at: enter your alarm time and get the bedtimes that line up with the end of a cycle.
• Go to bed at: enter when you'll get into bed and get the best times to set your alarm.
• Sleep now: one tap. It uses the current time on your phone.

What time should I go to bed? Bedtime chart

Find your wake-up time and read across. Every bedtime includes 15 minutes to fall asleep. For most adults, the 5-cycle column is the one to aim for.

More detail for common alarms: 6 AM, 7 AM, 8 AM, or how to wake up at 5 AM.

What time should I wake up? Wake-up chart

Find the time you'll get into bed and read across for alarm times that land at the end of a cycle.

Going to bed late? See if you go to bed at 11 PM or at midnight.

How many sleep cycles do you need?

For most adults, 5 cycles (7.5 hours) or 6 cycles (9 hours) is the target. That lines up with the adult guideline of 7 to 9 hours a night. Four cycles (6 hours) will get you through a short night, but it isn't enough to live on. Here's why 6 hours falls short.

Children need more, and their totals include naps. See sleep needs for every age or the bedtime chart for kids.

What happens during a sleep cycle

Each cycle moves through four stages:

1. N1, drifting off: a few minutes of very light sleep. Easy to wake from.
2. N2, light sleep: the biggest share of the night. Heart rate slows and body temperature drops.
3. N3, deep sleep: slow-wave sleep, hardest to wake from. Waking here causes the heavy, groggy feeling called sleep inertia.
4. REM sleep: most dreaming happens here. It's tied to memory and emotional processing.

The mix changes through the night. Early cycles are heavy on deep sleep; later cycles carry more REM. So cutting your night short costs you mostly REM. Read more: how many sleep cycles you need and what REM sleep does.

Why 90 minutes is an estimate

90 minutes is an average. Real cycles run anywhere from about 70 to 120 minutes, and they vary through the night and from person to person. So treat the results as good starting points, not exact times. If you keep waking up groggy at one of the suggested times, move your bedtime 15 minutes earlier or later for a few nights and see how you feel.

Tips to make your results work

• Keep your wake-up time the same every day, weekends included. It anchors your body clock more than anything else. (Is sleeping in on weekends bad?)
• Move bedtime in small steps. 15 minutes earlier every few days works better than a big jump. How to go to bed earlier.
• Watch the caffeine. An afternoon coffee can still be in your system at bedtime. Check your caffeine cutoff.
• Short on sleep this week? Work out your sleep debt and pay it back gradually.
• Need a boost during the day? Keep naps short. The nap calculator gives you the right alarm time.
• Work nights? The shift work calculator plans sleep around your shift.
• Want the whole day planned? The sleep schedule maker adds morning light, a caffeine cutoff, dinner and wind-down times to your bedtime.