Free Sleep Calculator

When a sleep calculator helps

The calculator is most useful when the alarm time is fixed and the bedtime is flexible: an early flight, a shift that starts at 6:00, a school run or a match the next morning. Working backwards from the wake-up time shows the latest bedtime that still fits a full night, and the bedtimes that line your alarm up with the end of a cycle rather than the middle of one.

It also helps in the other direction. If you are getting into bed late after a long day or a late shift, the bedtime mode shows which wake-up times land between cycles, so you can choose a later alarm that leaves you less groggy instead of a slightly earlier one that cuts into deep sleep. The I'm going to sleep now mode does the same thing using your device's current time.

Parents and carers can use the age-group panel to compare a child's actual hours against published recommendations, which include naps for babies and young children.

Worked example: a 6:30 alarm

Say you need to be up at 6:30, you usually take 15 minutes to fall asleep, and you keep the default 90-minute cycle. The calculator subtracts the 15 minutes and then whole cycles from the wake-up time. Six cycles (9 hours of sleep) means going to bed at 21:15 the evening before; five cycles (7 h 30 min) gives 22:45; four cycles (6 hours) gives 00:15; and three cycles (4 h 30 min) gives 01:45.

With the Adult (26–64) group selected, the National Sleep Foundation range is 7 to 9 hours, so the 21:15 and 22:45 options are highlighted as within the range, and the 00:15 and 01:45 options are marked as less than recommended. In 12-hour mode the same list reads 9:15 PM, 10:45 PM, 12:15 AM and 1:45 AM.

Now the reverse: going to bed at 23:00 with a 100-minute cycle and 10 minutes to fall asleep gives wake-up times of 04:10, 05:50, 07:30 and 09:10 the next day, for 5, 6 h 40 min, 8 h 20 min and 10 hours of sleep.

How the times and recommendations are worked out

Bedtime = wake-up time − time to fall asleep − cycles × cycle length. Wake-up time = bedtime + time to fall asleep + cycles × cycle length. Times wrap past midnight, and each result is labelled as the evening before, the same day or the next day so you never misread a 00:15 bedtime.

The age-group figures come from two expert consensus statements. The National Sleep Foundation (Hirshkowitz et al., Sleep Health, 2015) recommends 14–17 hours for newborns (0–3 months), 12–15 for infants (4–11 months), 11–14 for toddlers (1–2 years), 10–13 for preschoolers (3–5), 9–11 for school-age children (6–13), 8–10 for teenagers (14–17), 7–9 for young adults and adults (18–64) and 7–8 for adults 65 and older.

The American Academy of Sleep Medicine published 12–16 hours for ages 4–12 months, 11–14 for 1–2 years, 10–13 for 3–5 years, 9–12 for 6–12 years and 8–10 for 13–18 years (Paruthi et al., 2016), and 7 or more hours per night for adults 18–60 (Watson et al., 2015, with the Sleep Research Society). AASM makes no recommendation for babies under 4 months.

Common mistakes to avoid

Treating 90 minutes as exact is the most common one. Cycles vary from person to person and across the night, with more deep sleep early and more REM sleep later, so a suggested time is a good target rather than a precise moment. If you consistently wake before the alarm, try a slightly shorter cycle.

Another mistake is forgetting the time to fall asleep. Getting into bed at 22:45 does not mean sleep starts at 22:45. Finally, cycle timing never makes up for too little sleep: waking between cycles after four cycles still leaves an adult well short of the recommended seven hours.