Sleep occurs in cycles of approximately 90 minutes. Each cycle includes light sleep, deep sleep, and REM sleep. Waking up at the end of a cycle (rather than in the middle of deep sleep, which causes grogginess known as sleep inertia) helps you feel more refreshed. This calculator finds optimal sleep and wake times based on complete 90-minute cycles, accounting for the average 14 minutes it takes to fall asleep. The 90-minute figure is an average; real cycles range from roughly 70 to 120 minutes and lengthen over the night, so treat the suggested bedtimes as a guide rather than a precise rule. This information is for general reference and is not medical advice.
Recommended Sleep Duration
Age Group
Recommended Hours
Newborn (0-3 months)
14-17 hours
Infant (4-12 months)
12-16 hours (incl. naps)
Toddler (1-2 years)
11-14 hours (incl. naps)
Preschool (3-5 years)
10-13 hours (incl. naps)
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
Older adult (65+ years)
7-8 hours
Source U.S. Centers for Disease Control and Prevention (CDC) recommended sleep duration by age.
Frequently Asked Questions
Q: How long is one sleep cycle?
A: About 90 minutes on average, cycling through light sleep, deep sleep, and REM. Individual cycles vary from roughly 70 to 120 minutes, so this calculator uses the common 90-minute average.
Q: How much sleep does an adult need?
A: The CDC recommends 7 or more hours per night for adults aged 18-60. Teens need 8-10 hours and school-age children 9-12 hours.
Q: Why do I sometimes feel groggy even after 8 hours?
A: Waking in the middle of a deep-sleep stage can cause grogginess (sleep inertia). Aiming to wake at the end of a cycle, plus consistent timing and good sleep habits, can help you feel more refreshed.