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Sleep Calculator
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Sleep Science

One Cycle = 90 Minutes

Wake at the end of a cycle and feel refreshed. Wake in the middle and feel groggy. That's it.

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90m

Per Cycle

5–6

Ideal Cycles

20%

Deep Sleep

25%

REM Sleep

One Cycle, Four Stages

Light
Deep
REM
Awake N1 N2 N3 N1 — Light sleep entry, ~5% of total sleep N2 — Core light sleep with sleep spindles N3 — Deep sleep, growth hormone release Brief N2 — Transition before REM REM — Dreaming, memory consolidation N1 ~5 min N2 Light · ~25 min N3 Deep · ~30 min REM Dreams · ~20 min 0 20 min 45 min 65 min 90 min
waves

Light Sleep

~50%

Heart rate slows, muscles relax. Sleep spindles filter noise and protect your sleep.

bolt

Deep Sleep

~20%

Growth hormone release, tissue repair. Waking here causes the worst grogginess.

psychology

REM Sleep

~25%

Dreams, memory consolidation, emotional processing. Gets longer each cycle.

Why Timing Matters

alarm_off

Wake Mid-Cycle

Deep Sleep Interrupted

Grogginess, confusion, impaired performance for 30+ minutes. Your brain was in its slowest wave state.

alarm_on

Wake at Cycle End

Light Sleep Transition

Alert and refreshed within minutes. You're already close to consciousness — waking feels effortless.

Ready to sleep smarter?

Use our calculator to find bedtimes and wake times aligned with your natural sleep cycles.

Open Sleep Calculator or listen to sleep sounds →

Nap Science

The Post-Lunch Alertness Dip

A natural drop in alertness between 1–4 PM. A short nap during this window boosts cognitive performance and mood.

26%

Alertness Boost

20min

Optimal Duration

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High Low 9AM 12PM 3PM 6PM NAP WINDOW
menu_book Sources & References expand_more

Walker, M. (2017)

Why We Sleep: Unlocking the Power of Sleep and Dreams. Scribner.

National Institute of Neurological Disorders and Stroke

Brain Basics: Understanding Sleep — ninds.nih.gov

Rosekind, M. R. et al. (1995)

Alertness management: strategic naps in operational settings. J. Sleep Research.

FAQ

What is a 90-minute sleep cycle? expand_more

A sleep cycle lasts approximately 90 minutes and passes through four stages: N1 (light entry, ~5 min), N2 (true sleep with sleep spindles, ~25 min), N3 (deep sleep with growth hormone release, ~30 min), and REM (dreaming and memory consolidation, ~20 min). You go through 5–6 of these cycles per night.

Why do I feel groggy when I wake up? expand_more

That grogginess is called sleep inertia — it happens when your alarm goes off during deep sleep (N3 stage). Your brain is in its slowest wave state and needs 30+ minutes to fully wake up. Waking at the end of a cycle, during light sleep, avoids this entirely.

What happens during REM sleep? expand_more

REM (Rapid Eye Movement) sleep is when you dream, consolidate memories, and process emotions. It makes up about 25% of total sleep. REM periods get longer as the night goes on — your first REM might be 10 minutes, but the last one before waking can be 60 minutes.