How the calculator works it out
Sleep runs in cycles of light sleep, deep sleep and REM sleep. A cycle averages about 90 minutes, and most people go through 4 to 6 of them a night (StatPearls, Physiology, Sleep Stages). Waking near the end of a cycle, in lighter sleep, is thought to feel easier than waking from deep sleep. The calculator also allows for the time it takes to drop off, which is about 10 to 20 minutes for most adults (Sleep Foundation).
Bedtime = wake-up time − time to fall asleep − cycles × cycle length
Wake-up time = bedtime + time to fall asleep + cycles × cycle length
Sleep = cycles × cycle length (6 cycles = 9 h, 5 = 7.5 h, 4 = 6 h, 3 = 4.5 h)
Each option is then checked against the hours the CDC recommends for the age group you picked. The suggested time is the latest bedtime, or the earliest wake-up, that still meets that range.
Worked example
You need to be up at 6:30 AM. Take off 15 minutes to fall asleep and you need to be asleep by 6:15 AM. Five cycles of 90 minutes is 7 hours 30 minutes, so the bedtime is 6:15 AM − 7:30 = 10:45 PM. Six cycles (9 hours) means 9:15 PM and four cycles (6 hours) means 12:15 AM. For an adult aged 18 to 60, 10:45 PM is the latest bedtime that gives the recommended 7 or more hours.
Recommended sleep by age
| Age | Hours per day |
|---|---|
| Newborn, 0–3 months | 14–17 hours, including naps |
| Infant, 4–12 months | 12–16 hours, including naps |
| Toddler, 1–2 years | 11–14 hours, including naps |
| Preschool, 3–5 years | 10–13 hours, including naps |
| School age, 6–12 years | 9–12 hours |
| Teen, 13–17 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 |
Source: CDC, About Sleep, which draws on the American Academy of Sleep Medicine consensus statements for adults (Watson et al., 2015) and children (Paruthi et al., 2016) and on the National Sleep Foundation (Hirshkowitz et al., 2015).
Limits
- Cycle length varies between people and across the night. Early cycles hold more deep sleep and later ones more REM sleep. The times here are a guide, not a precise forecast.
- Total sleep matters more than timing. If a cycle-friendly bedtime would cut your sleep short, the earlier bedtime is the better choice.
- For babies and young children the recommended hours include naps, so night-time cycles alone do not cover them.
- Snoring, gasping, or feeling sleepy during the day despite enough sleep are reasons to see a doctor. This page does not give medical advice.
Questions people ask
What time should I go to bed to wake up at 6:30 AM?
Allowing about 15 minutes to fall asleep and 90-minute cycles, the bedtimes that end on a full cycle are 9:15 PM (6 cycles, 9 hours), 10:45 PM (5 cycles, 7.5 hours) and 12:15 AM (4 cycles, 6 hours). For most adults 10:45 PM is the latest that still gives the 7 or more hours the CDC recommends.
Is a sleep cycle really 90 minutes?
About 90 minutes is the average, but it is not fixed. Cycles vary from person to person and through the night; StatPearls puts a full cycle at roughly 90 to 110 minutes, with 4 to 6 cycles a night. Treat the times here as a starting point, and change the cycle length if you find you wake more easily at a different time.
How much sleep do I need?
The CDC, based on American Academy of Sleep Medicine and National Sleep Foundation recommendations, says adults 18 to 60 need 7 or more hours, 61 to 64 need 7 to 9 and 65 and older 7 to 8. Teens 13 to 17 need 8 to 10 hours and children 6 to 12 need 9 to 12. Younger children need more, including naps.
Why does it add 15 minutes?
Most adults take about 10 to 20 minutes to fall asleep, so the calculator adds 15 minutes by default before the first cycle starts. If you usually drop off faster or take longer, change it under "Adjust the timings".
Does waking at the end of a cycle stop grogginess?
It may help, because you are more likely to wake from light sleep than deep sleep. There is little direct evidence for exact cycle timing, though, and getting enough total sleep matters far more than hitting a cycle boundary. If you are often tired despite enough sleep, talk to your doctor.
Can I use it for naps?
It is built for overnight sleep, so it shows 3 to 6 cycles. For young children the CDC hours include naps, so a night of cycles on its own will look short for them; count daytime naps toward the total.