Sleep Calculator
Wake up refreshed by sleeping in complete 90-minute cycles. Never feel groggy again.
Enter your desired wake-up time and we'll show you the best times to go to sleep tonight.
Enter when you're going to sleep and we'll show you the best times to set your alarm.
Why 90 Minutes?
Light Sleep
The first ~30 min of each cycle. Your body temperature drops, heart rate slows. This is the easiest stage to wake up from without grogginess.
Deep Sleep
The hardest to wake from. Your body repairs tissue, builds bone and muscle. Waking during deep sleep causes sleep inertia — the "hit by a bus" feeling.
REM Sleep
Rapid Eye Movement — when most dreaming happens. Critical for memory consolidation, learning, and emotional regulation. Increases in later cycles.
| Cycles | Sleep Duration | Recommended for |
|---|---|---|
| 3 cycles | 4h 30m | Emergency only — not sustainable |
| 4 cycles | 6h 00m | Minimum for adults, short-term |
| 5 cycles ⭐ | 7h 30m | Optimal for most adults |
| 6 cycles | 9h 00m | Recovery, illness, teenagers |
Frequently Asked Questions
How long is a sleep cycle?
A full sleep cycle lasts approximately 90 minutes and includes light sleep, deep sleep (slow-wave), and REM sleep. Most adults complete 4–6 cycles per night. Waking up at the end of a cycle is what makes you feel refreshed.
How many hours of sleep do I need?
Adults need 7–9 hours (4–6 full cycles). Teenagers need 8–10 hours. The exact amount varies, but waking at the end of a cycle matters more than the raw total. 7h 30m (5 cycles) is the sweet spot for most adults.
Why do I wake up tired after 8 hours of sleep?
You likely woke up mid-cycle, during deep sleep. The grogginess is called sleep inertia. 8 hours = 5 cycles + 30 min extra, which lands you deep in cycle 6. Use this calculator to time your alarm at exactly 6 × 90 min = 9h, or 5 × 90 min = 7h 30m.
What time should I go to bed to wake up at 7am?
To wake up at 7:00 AM refreshed, go to sleep at: 9:45 PM (6 cycles, 9h15m) • 11:15 PM (5 cycles, 7h45m) ⭐ • 12:45 AM (4 cycles, 6h15m) • 2:15 AM (3 cycles, 4h45m). All times include 15 min to fall asleep.
Is it better to sleep 6 hours or 7.5 hours?
7h 30m is better — and often you'll feel MORE rested than after 8 hours. 6h (4 cycles) is a clean cycle boundary, so it's better than 6h 30m. But if you can get 7h 30m (5 cycles), that's the scientifically recommended target for adults.