bedtimeHealth

Sleep Calculator

Wake up refreshed by timing your sleep to complete 90-minute cycles.

I need to wake up at

Includes ~14 min to fall asleep
I'm going to sleep at

Includes ~14 min to fall asleep

Understanding Sleep Cycles

N1 — Light Sleep (5 min)
The transition into sleep. Easy to wake, muscle twitches common. Eye movement slows.
N2 — Core Sleep (25 min)
True sleep begins. Heart rate slows, body temperature drops. Sleep spindles occur. ~50% of total sleep time.
N3 — Deep Sleep (30 min)
Hardest to wake from. Physical repair, immune function, and memory consolidation happen here.
REM — Dream Sleep (30 min)
Rapid eye movement. Critical for emotional regulation, creativity, and long-term memory. Gets longer in later cycles.

Frequently Asked Questions

A complete sleep cycle lasts approximately 90 minutes and includes N1 (light), N2 (core), N3 (deep), and REM stages. Adults typically complete 4–6 cycles per night. Waking at the end of a cycle feels far more refreshing than waking mid-cycle, particularly from deep N3 sleep.
The CDC recommends 7–9 hours for adults aged 18–64 (7–8 hours for 65+). This is 5–6 complete cycles. Consistently sleeping fewer than 6 hours impairs cognition, immune function, and metabolic health, even if you "feel fine" due to chronic sleep debt adaptation.
Waking during deep N3 sleep causes sleep inertia — the groggy feeling that lasts 15–60 minutes. If your alarm interrupts a cycle mid-way, you may feel worse than after 7.5 hours at a cycle boundary. Timing your wake-up to a cycle end dramatically reduces this effect.
The average sleep-onset latency (time from lying down to falling asleep) is 10–20 minutes for healthy adults. This calculator uses 14 minutes as a reasonable middle estimate. If you typically fall asleep faster or slower, your optimal bedtime will differ slightly.

About the Sleep Calculator

Calculate the optimal times to wake up or go to sleep based on 90-minute sleep cycles. Enter your target wake time to see when to go to bed, or enter your bedtime to see the best wake times that avoid waking mid-cycle. Waking at the end of a cycle instead of the middle of one significantly reduces sleep inertia and morning grogginess.

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