Most adults need 7 to 9 hours of sleep each night. Teenagers need 8 to 10 hours. School-age children need 9 to 12 hours. Newborns need 14 to 17 hours, including naps.

Top guides list the same age table but rarely show how to turn it into a bedtime. They also skip the arithmetic of sleep cycles and sleep debt. This guide gives the table, then works out a real bedtime, a cycle count, and a debt total you can check.

Use the National Sleep Foundation ranges as your starting point. Newborns aged 0 to 3 months need 14 to 17 hours. Infants aged 4 to 11 months need 12 to 15 hours. Toddlers aged 1 to 2 years need 11 to 14 hours. Preschoolers aged 3 to 5 years need 10 to 13 hours. School-age children aged 6 to 13 years need 9 to 11 hours. Teenagers aged 14 to 17 years need 8 to 10 hours. Adults aged 18 to 64 years need 7 to 9 hours. Older adults aged 65 and over need 7 to 8 hours.

Worked example: find your bedtime from your wake time. Suppose you must wake at 6:30 in the morning and you need 8 hours of sleep. Subtract 8 hours from 6:30 and you get 10:30 at night. Now allow 15 minutes to fall asleep. Move bedtime 15 minutes earlier, to 10:15 at night. That is the time to be in bed with the lights off, not the time to start getting ready.

Check the same night in sleep cycles. Sleep runs in cycles of about 90 minutes. Five cycles last 5 times 90, which is 450 minutes, or 7.5 hours. Six cycles last 6 times 90, which is 540 minutes, or 9 hours. An 8-hour night sits between five and six full cycles, which is why waking in the middle of a cycle can feel groggy even after enough total hours.

Count sleep debt the same way. If you need 8 hours and sleep 7 hours for five nights, you lose 1 hour each night. Five nights times 1 hour equals 5 hours of sleep debt. You cannot repay 5 hours with one long lie-in, because one night of extra sleep rarely adds more than 1 to 2 hours. Repay it with an extra hour a night for about five nights instead.

Judge your own number by how you feel, not by the table alone. You are probably getting enough sleep if you wake without an alarm, stay alert through the afternoon without caffeine, and do not fall asleep within minutes of sitting down. You are probably short if you need an alarm every day, feel drowsy by mid-afternoon, or fall asleep the moment your head touches the pillow.

Keep the same bedtime and wake time for a week before changing anything. A fixed wake time matters more than a fixed bedtime, because it anchors your body clock. Move bedtime earlier by 15 minutes at a time until you wake rested. Changing the time by an hour in one night rarely works.

This is general information, not medical advice. Sleep problems that last for weeks, loud snoring with pauses in breathing, or severe daytime sleepiness belong with a doctor.

Q: How much sleep does an adult need each night?

A: Most adults need 7 to 9 hours of sleep each night, according to the National Sleep Foundation. Adults aged 65 and over need 7 to 8 hours.

Q: How do I work out my bedtime?

A: Subtract your sleep need from your wake time, then move 15 minutes earlier to allow time to fall asleep. A 6:30 wake time with an 8-hour need gives a 10:15 bedtime.

Q: How long is one sleep cycle?

A: About 90 minutes. Five cycles last 7.5 hours and six cycles last 9 hours.

Q: Can I repay sleep debt in one night?

A: Usually not. One long night rarely adds more than 1 to 2 hours, so repay a 5-hour debt with an extra hour a night for about five nights.