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Sleep Calc

Understanding Your
90-Minute Sleep Cycle

A deep dive into the biological phases that restore your body and mind. Our methodology combines chronobiology with modern sleep hygiene research.

The Sleep Architecture

Typical progression of a single healthy sleep cycle

Light
Deep
REM
N1 — Light Sleep entry, ~5% of total sleep. Lasts only a few minutes as you drift off.
N1
N2 — Core light sleep, ~45% of total sleep. Sleep spindles protect against noise and aid memory.
N2
N3 — Deep sleep, ~15–20% of total sleep. Growth hormone release, tissue repair, immune boost.
N3
N2 — Brief return to light sleep before REM begins. Transition phase.
N2
REM — Rapid Eye Movement, ~20–25% of total sleep. Dreaming, emotional processing, memory consolidation.
REM
0 Minutes 45 Minutes 90 Minutes
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Light Sleep

(N1 & N2)

Makes up about 50–60% of your total sleep. Your heart rate slows, body temperature drops, and muscles relax. N2 produces sleep spindles — bursts of brain activity that help filter out external noise and protect your sleep.

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Deep Sleep

(N3)

The "Physical Repair" stage, accounting for 15–20% of sleep. Human Growth Hormone is released to repair tissues and strengthen the immune system. Waking during this phase causes the worst sleep inertia — that heavy, disoriented feeling.

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REM Sleep

The "Mental Cleanup" stage, making up 20–25% of sleep. Your brain processes emotions, consolidates memories, and clears metabolic waste. REM periods get longer with each cycle — the last cycle before waking has the most REM.

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Why Timing Your Wake-Up Matters

The core principle behind every calculation on this site

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Waking mid-cycle (Deep Sleep)

Your brain is in its slowest wave state. Being forced awake here causes sleep inertia — grogginess, confusion, and impaired performance that can last 30+ minutes.

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Waking at cycle end (Light Sleep)

Each cycle naturally returns to light sleep before the next begins. Waking here feels effortless — you're already close to consciousness, alert and refreshed within minutes.

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The Post-Lunch Alertness Dip

The Post-Prandial Dip is a natural drop in alertness that occurs between 1–4 PM, influenced by both your circadian rhythm and digestive processes. A short nap during this window can significantly boost cognitive performance and mood.

26%

Alertness Boost

20min

Optimal Duration

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High Low 9AM 12PM 3PM 6PM NAP WINDOW
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Methodological Sources

Walker, M. (2017)

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

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National Institute of Neurological Disorders and Stroke

Brain Basics: Understanding Sleep — ninds.nih.gov

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

Alertness management: strategic naps in operational settings. Journal of Sleep Research, 4(S2), 62–66.

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