How the sleep calculator works
Sleep is not one long, even state. Through the night you move through repeating sleep cycles: light sleep, then deep (slow-wave) sleep, then REM sleep, when most vivid dreaming happens. Each cycle lasts about 90 minutes on average. Early cycles have more deep sleep; later ones have longer REM periods.
If your alarm goes off in the middle of deep sleep you are more likely to wake up groggy. Waking near the end of a cycle, when sleep is lighter, tends to feel easier. So the calculator plans sleep in whole cycles and adds the time it takes you to drift off.
Wake-up time = bedtime + time to fall asleep + (cycles × 90 min)
It shows options for 6, 5, 4 and 3 cycles, which is 9, 7.5, 6 and 4.5 hours of actual sleep. Five or six cycles are marked as recommended because they meet the 7-hour minimum for adults.
Worked examples
Example 1: you need to be up at 6:30 a.m. With 15 minutes to fall asleep:
- 6 cycles: 6:30 − 15 min − 9 h = 9:15 p.m.
- 5 cycles: 6:30 − 15 min − 7 h 30 min = 10:45 p.m.
- 4 cycles: 6:30 − 15 min − 6 h = 12:15 a.m. (a short night)
So the sensible choices are 9:15 or 10:45 p.m.
Example 2: you are going to bed at 11:30 p.m. Add 15 minutes to fall asleep, then whole cycles:
- 5 cycles: 11:30 p.m. + 15 min + 7 h 30 min = 7:15 a.m.
- 6 cycles: 11:30 p.m. + 15 min + 9 h = 8:45 a.m.
If you must be up by 7:00, it is better to go to bed 15 minutes earlier than to cut a cycle short.
Example 3: you take longer to fall asleep. If it usually takes you 30 minutes, change that box to 30. For a 7:00 a.m. alarm and five cycles, bedtime moves from 11:15 p.m. to 11:00 p.m.
How much sleep you need by age
The American Academy of Sleep Medicine (AASM) published consensus recommendations for children in 2016 and, with the Sleep Research Society, for adults in 2015. The CDC uses the same figures. Children’s totals include naps.
| Age | Recommended sleep per 24 hours |
|---|---|
| Newborn, 0–3 months | 14–17 hours (National Sleep Foundation) |
| Infant, 4–12 months | 12–16 hours |
| Toddler, 1–2 years | 11–14 hours |
| Preschool, 3–5 years | 10–13 hours |
| School age, 6–12 years | 9–12 hours |
| Teen, 13–18 years | 8–10 hours |
| Adult, 18–60 years | 7 or more hours |
| Adult, 61–64 years | 7–9 hours |
| Adult, 65 and older | 7–8 hours |
Regularly sleeping less than 7 hours as an adult is linked with weight gain, high blood pressure, diabetes, heart disease, depression and more accidents. Hitting the end of a cycle does not make a short night healthy; the total still matters most.
Cycles at a glance
| Cycles | Time asleep | Time in bed (with 15 min to fall asleep) |
|---|---|---|
| 3 | 4 h 30 min | 4 h 45 min |
| 4 | 6 h | 6 h 15 min |
| 5 | 7 h 30 min | 7 h 45 min |
| 6 | 9 h | 9 h 15 min |
Why your timing may need tweaking
Ninety minutes is an average. Real cycles vary between people and through the night, often from about 80 to 110 minutes, and the first one is sometimes shorter. Alcohol, caffeine, stress and noise also change how you sleep. If you still wake up groggy:
- Shift by 15 minutes. Move your bedtime earlier or later in 15-minute steps for a week and see what feels best.
- Try a different cycle length. Set it to 85 or 100 minutes and compare.
- Check the total. Feeling tired every morning often means you simply need more sleep, not better timing.
Tips for better sleep
These are based on NHS and NIH advice:
- Keep a regular schedule. Go to bed and get up at about the same time every day, including weekends. This matters more than any single night.
- Get morning daylight. Light early in the day helps set your body clock.
- Cut caffeine after lunch. Caffeine can stay in your system for many hours.
- Be careful with alcohol. It can help you fall asleep but breaks up sleep later in the night.
- Make the bedroom dark, quiet and cool. Put screens away for the last half hour before bed.
- Do not lie awake for ages. If you cannot sleep after about 20 minutes, get up, do something calm in dim light, and go back when sleepy.
See a doctor if poor sleep lasts more than a few weeks, you snore loudly or stop breathing in your sleep, or you are very sleepy in the day despite enough time in bed. These can be signs of insomnia or sleep apnoea, which are treatable.
Related calculators
Shift workers can total their hours with the hours calculator or add up times with the time calculator. If you are training, sleep is when you recover: plan your runs with the pace calculator and your fuel with the calorie calculator.