90 Minute Sleep Cycle Calculator: Best Bedtime and Wake-Up

Bedtime and wake-up time based on 90-minute sleep cycles

The calculator adds or subtracts 90-minute sleep cycles plus a time to fall asleep (14 minutes by default, adjustable). For a 7:00 AM alarm it suggests going to bed at 11:16 PM for 5 cycles (7.5 hours of sleep) or at 9:46 PM for 6 cycles (9 hours). Two more tabs complete the calculation: one estimates your sleep debt over the last 7 nights, the other gives nap length guides (20 or 90 minutes).

The calculation

• "I need to wake up at" mode: bedtime = wake time − cycles × 90 min − time to fall asleep, for 6, 5, 4 and 3 cycles.
• "I'm going to bed at" mode: wake time = bedtime + time to fall asleep + cycles × 90 min.
• Times wrap past midnight; the 5-cycle row (7.5 hours) is highlighted.

Example

Alarm at 7:00 AM with 14 minutes to fall asleep: go to bed at 9:46 PM (6 cycles), 11:16 PM (5 cycles), 12:46 AM (4 cycles) or 2:16 AM (3 cycles). The other way round, in bed at 11:00 PM you would wake at the end of a cycle at 5:14 AM, 6:44 AM or 8:14 AM.

Sleep debt over 7 nights

• In the sleep debt tab you enter your nightly need (8 hours by default, adjustable from 5 to 12 hours in 0.5-hour steps) and the hours slept on each of the last 7 nights.
• Debt = need × 7 − total hours slept; the nightly average is total ÷ 7. Total, debt and average are rounded to one decimal.
• Zero or negative debt: no deficit, shown as +0 h (a surplus is not shown); up to 4 hours: mild debt; above that: high debt.

With the default values (8-hour need, nights of 7, 6.5, 7.5, 6, 8, 8.5 and 7 hours) you slept 50.5 hours against 56 targeted: the debt is 5.5 hours and the average 7.2 hours a night, which counts as high debt. This is an estimate based on a plain difference in hours: it ignores sleep quality and how long the shortfall has lasted, and the 8-hour need is a default you should adapt.

Naps

The nap tab shows three fixed reference lengths. A 20-minute nap stays in light sleep. A 90-minute nap covers a full cycle. The 45 to 60-minute window is best avoided, as you may then wake in deep sleep.

Assumptions and limits

The 90-minute cycle is an average: according to France's Inserm, a night holds 3 to 6 cycles of 60 to 120 minutes, and their length varies between people and nights. The tool still uses a fixed 90 minutes and the same time to fall asleep every evening, so the suggested times are guides, not a guarantee of waking outside deep sleep. For adults, the AASM and the Sleep Research Society recommend at least 7 hours of sleep a night, and Inserm speaks of a 7-hour minimum: the 4-cycle (6 hours) and 3-cycle (4.5 hours) options fall short of it. If you regularly need more than 30 minutes to fall asleep, or stay tired despite long enough nights, talk to a doctor.

Sources and dates: Inserm, "Sommeil" dossier (page updated 08/11/2024): 3 to 6 cycles of 60 to 120 minutes per night, "at least 3 to 5 cycles of 90 minutes on average" according to the caption of its illustration, and a 7-hour daily minimum. Watson et al., Sleep, 2015, consensus of the AASM and the Sleep Research Society: at least 7 hours a night for adults. The 14-minute sleep onset is the tool's own value. The 8-hour default need, the 4-hour debt threshold and the nap lengths are the tool's own guides, not a standard (checked 14/10/2026; Inserm and AASM rechecked 28/10/2026).

Frequently Asked Questions

What time should I go to bed to wake up at 7:00 AM?

With 90-minute cycles and 14 minutes to fall asleep, the tool suggests 11:16 PM for 5 cycles (7.5 hours of sleep) or 9:46 PM for 6 cycles (9 hours). If you fall asleep faster or slower, adjust the sleep-onset time: each minute moves the bedtime by the same amount.

How long is a sleep cycle?

About 90 minutes on average, the length the tool uses. France's Inserm describes a night of 3 to 6 cycles lasting 60 to 120 minutes, each made of non-REM sleep followed by REM sleep. The exact length varies from person to person.

How many sleep cycles do I need per night?

The tool highlights 5 cycles (7.5 hours of sleep) and also offers 6 cycles (9 hours). 4 cycles (6 hours) and 3 cycles (4.5 hours) fall short of the 7 hours a night recommended for adults (AASM/SRS 2015, Inserm). Inserm counts 3 to 6 cycles depending on the night.

Why am I tired even after a long night?

Waking from deep sleep causes a groggy feeling called sleep inertia; aiming for the end of a cycle can reduce it. If tiredness persists despite 7 to 9-hour nights, it may have a medical cause that a doctor can look for.

How do I calculate my sleep debt?

Multiply your nightly need by 7, then subtract the total hours you actually slept that week. With an 8-hour need (56 hours) and 50.5 hours slept, the debt is 5.5 hours; the tool rates it "high" above 4 hours.

What is the ideal nap length?

The tool suggests 20 minutes to stay in light sleep, or 90 minutes to cover a full cycle. It advises against 45 to 60 minutes, which can wake you in deep sleep and leave you groggier than before.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.