How it's calculated
During the night, sleep moves through repeating cycles of lighter, deep and REM sleep. Each cycle lasts roughly 90 minutes. Waking at the end of a cycle, rather than in deep sleep, may help you feel less groggy. Most people take 10 to 20 minutes to fall asleep.
wake time = bedtime + time to fall asleep + cycles × 90 min
This is a rule of thumb, not a measurement. Cycles vary from about 70 to 120 minutes, and they change through the night.
Worked example
You want to wake at 7:00 AM and take 15 minutes to fall asleep. For 6 cycles (9 hours): 7:00 AM − 15 min − 9 h = 9:45 PM. For 5 cycles (7½ hours): 11:15 PM. For 4 cycles (6 hours): 12:45 AM, which is less than adults need.
How much sleep do you need?
| Age group | Recommended sleep per day |
|---|---|
| Newborns (0–3 months) | 14–17 hours |
| Infants (4–12 months) | 12–16 hours, including naps |
| Toddlers (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 |
| Teens (13–17 years) | 8–10 hours |
| Adults (18–60 years) | 7 or more hours |
| Adults (61–64 years) | 7–9 hours |
| Adults (65 and older) | 7–8 hours |
These are the CDC's recommendations, based on the American Academy of Sleep Medicine (children and adults to 60) and the National Sleep Foundation (newborns and adults over 60).
Limitations
- Total sleep matters more than timing your alarm to a cycle. A regular schedule helps most people.
- Shift work, jet lag, new babies, sleep apnea and insomnia can all change how sleep cycles fit together.
- Regularly sleeping much more or less than the recommended range can be a reason to talk to a doctor.
Sources
- CDC: About Sleep, recommended hours by age
- Paruthi S et al. Recommended amount of sleep for pediatric populations: AASM consensus statement. J Clin Sleep Med. 2016;12(6):785–786
- Watson NF et al. Recommended amount of sleep for a healthy adult: AASM and SRS joint consensus statement. J Clin Sleep Med. 2015;11(6):591–592
- NIH, National Institute of Neurological Disorders and Stroke: Brain Basics: Understanding Sleep sleep stages; REM sleep first occurs about 90 minutes after falling asleep.