😴 Sleep Cycles 90-min cycles Universal
Sleep Calculator
Stop waking up groggy. Sleep in 90-minute cycles to wake up feeling rested. Enter your wake-up time or bedtime to get optimal sleep/wake windows.
Quick Answer
To wake at 7:00 AM feeling rested → go to sleep at 11:16 PM (7.5 hrs) or 9:46 PM (9 hrs). Each sleep cycle = 90 minutes. Add 14 mins to fall asleep. Waking mid-cycle causes grogginess. The magic number is 5 cycles (7.5 hours) for most adults.
Sleep Recommendations by Country Health Authority
| Age Group | CDC (US) | NHS (UK) | WHO |
|---|---|---|---|
| Newborns (0–3 months) | 14–17 hrs | 14–17 hrs | 14–17 hrs |
| School-age (6–12 years) | 9–12 hrs | 9–11 hrs | 9–11 hrs |
| Teens (13–18 years) | 8–10 hrs | 8–10 hrs | 8–10 hrs |
| Adults (18–64 years) | 7–9 hrs | 7–9 hrs | 7–9 hrs |
| Older adults (65+) | 7–8 hrs | 7–8 hrs | 7–8 hrs |
Sources: CDC 2022, NHS UK, WHO Global Status Report on NCDs.
Frequently Asked Questions
What time should I sleep to wake up at 6:30 AM? ▾
To wake at 6:30 AM, go to sleep at: 9:16 PM (9 hrs / 6 cycles), 10:46 PM (7.5 hrs / 5 cycles — ideal), 12:16 AM (6 hrs / 4 cycles), or 1:46 AM (4.5 hrs / 3 cycles). These times include 14 minutes to fall asleep.
Is 6 hours of sleep enough? ▾
For most adults, 6 hours is consistently below the recommended minimum of 7 hours. Short-term: cognitive performance drops significantly. Long-term: research links chronic sleep deprivation under 6 hours with increased risk of type 2 diabetes, cardiovascular disease, obesity, and immune dysfunction. However, 6 hours at the end of a complete cycle (e.g., waking at the end of cycle 4) is better than 6.5 hours waking mid-cycle.
Why do I feel more tired after 8 hours than 7.5 hours? ▾
Because you woke mid-cycle. 8 hours from falling asleep doesn't land at a cycle boundary — the nearest boundaries are 7.5 hours (5 cycles) and 9 hours (6 cycles). Waking during deep N3 sleep (which occurs most often in cycles 1–3) causes the worst sleep inertia. Use this calculator to align your alarm with a cycle end.
Do sleep schedules differ by country? ▾
Yes significantly. A 2016 study (University of Michigan, 50,000 participants in 100 countries) found: Spain averages midnight bedtime. Japan averages 12:11 AM. UK and Germany: 11:30 PM. USA: 11:58 PM. Singapore: 12:09 AM. The study found that social pressure and light exposure drive these differences more than biological factors.