The short answer
For adults ages 18 to 64, the widely published recommendation is 7 to 9 hours of sleep per night. For adults 65 and older, it is 7 to 8 hours. Children and teenagers need more, and the younger you are, the more sleep the guidelines recommend.
These ranges come from sleep-medicine consensus panels that reviewed hundreds of studies linking sleep duration to health, attention, and mood. They are guidelines for general education, not medical advice for any individual. If you consistently feel exhausted despite sleeping in the recommended range, that is a conversation for a doctor, not a calculator.
Recommended sleep by age
Here are the commonly published nightly ranges, including naps for the youngest groups:
Newborns (0 to 3 months): 14 to 17 hours. Infants (4 to 12 months): 12 to 16 hours, including naps. Toddlers (1 to 2 years): 11 to 14 hours, including naps. Preschoolers (3 to 5 years): 10 to 13 hours, including naps. School-age children (6 to 12 years): 9 to 12 hours. Teenagers (13 to 17 years): 8 to 10 hours. Adults (18 to 64 years): 7 to 9 hours. Adults 65 and older: 7 to 8 hours.
Notice the pattern: the range narrows and drops as the brain matures. A teenager who thrives on 9 hours is not sleeping too much; that is squarely inside the 8 to 10 hour band for ages 13 to 17.
Hours to sleep cycles: the math
Sleep moves in roughly 90-minute cycles, so it helps to convert hours into cycles. Multiply hours by 60, then divide by 90.
7.5 hours is 450 minutes, and 450 divided by 90 is exactly 5 cycles. 9 hours is 540 minutes, divided by 90 is exactly 6 cycles. That is why the adult recommendation of 7 to 9 hours maps so neatly onto 5 to 6 full sleep cycles: the two numbers are the same idea in different units.
Not every total lands on a whole cycle. 8 hours is 480 minutes, and 480 divided by 90 is 5.33 cycles, meaning you would wake about a third of the way into your sixth cycle. In practice, 8 hours still works well for most people, because cycle length varies a bit from night to night. The guideline cares about total hours; the cycle view just helps you time your alarm so it lands near the end of a cycle instead of in the middle of deep sleep.
Worked example: moving from 6 hours to 7.5
Say you currently sleep from midnight to 6:00 AM, which is 6 hours, or 4 full cycles. You want to reach the bottom of the adult range, 7.5 hours, or 5 full cycles.
The gap is 7.5 minus 6, which is 1.5 hours, exactly one 90-minute cycle. Shifting bedtime from midnight to 10:30 PM adds that full cycle: asleep at 10:45 after the usual 15-minute fall-asleep buffer, five cycles of 90 minutes is 450 minutes, and 10:45 PM plus 7 hours 30 minutes is 6:15 AM. Adjust the alarm to 6:15, or keep the 6:00 alarm and go to bed at 10:15 PM.
The key insight is that adding a full 90-minute cycle beats adding a random 30 or 45 minutes. A 6-hour-45-minute night ends mid-cycle, while a 7.5-hour night ends between cycles, and the between-cycle wake usually feels noticeably easier.
Worked example: a teenager at 9 hours
A 15-year-old sleeping 9 hours gets 540 minutes of sleep. 540 divided by 90 is exactly 6 full cycles, a textbook night inside the 8 to 10 hour teen range. At 10 hours (600 minutes), that is 6.67 cycles, also fine: the extra time beyond the sixth cycle usually lands in light sleep or REM near morning, which is the easiest part of the night to wake from.
Sleep debt: the weekly math
Sleep debt is the gap between what you need and what you get, added up over nights. Say you need 8 hours but get 6.5 hours on each of 5 weeknights. The nightly shortfall is 1.5 hours, times 5 nights, which is 7.5 hours of debt by Friday.
Repaying it takes time. Adding 1 extra hour per night clears 7.5 hours in about 7 to 8 nights, and most sleep researchers suggest paying it back gradually rather than with one marathon 12-hour sleep, which can leave you groggy and shift your schedule. Weekend catch-up helps, but it does not fully erase a week of short nights, which is why consistent bedtimes beat weekend recovery.
General signs you may need more sleep
Feeling groggy for an hour or more after waking, needing caffeine to function before noon, falling asleep quickly in quiet moments, and noticing your mood or focus dip are common signals that your sleep total is short of your need. None of these diagnose anything on their own, and persistent daytime sleepiness despite adequate hours is worth discussing with a clinician. The numbers in this guide are a starting point for planning your schedule, not a medical assessment.
Put your number to work
Once you know your target, timing matters as much as the total. A sleep cycle calculator turns your wake-up time into bedtime options (or your bedtime into alarm options) so the hours you spend in bed actually land as whole cycles.