Zenthya · Science / Modern Science

Sleep is not a switch.
It's a cycle.

4–6 cycles a night. About 90 minutes each. Deep sleep early, REM late.

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Chapter 01

The night in cycles

Light → deep → REM, then again.

Hypnogram: a typical night from 11 PM to 7 AM A step line moves between Awake, REM, N1, N2 and N3 across the night. The first 90 minutes are shaded: sleep drops quickly into deep N3 sleep. Deep sleep is concentrated in the first half of the night; REM periods, drawn in the accent colour, get longer toward morning. Illustrative, not patient data. THE FIRST 90 MINUTESAwakeREMN1N2N311 PM1 AM3 AM5 AM7 AM Illustrative · REM in colour

Timing matters as much as total hours.

Chapter 02

The inner clock

One 24-hour rhythm runs it all.

The body's 24-hour rhythm A 24-hour clock with midnight at the top. The 9 to 11 PM arc, the Hour of Rest, is highlighted. Melatonin begins to rise about two hours before usual bedtime; deep sleep dominates the first half of the night; core temperature is lowest around 4 to 6 AM; alertness climbs mid-morning. 06121812349–11 PMTHE HOUR OF REST
  1. 1Melatonin begins to rise · ~2 h before usual bedtime
  2. 2Deep sleep dominates · first half of night
  3. 3Core temperature low · ~4–6 AM
  4. 4Alertness climbs · mid-morning
  5. The Hour of Rest · 9–11 PM

What the clock coordinates

  1. Sleep
  2. Hormones
  3. Body temperature
  4. Alertness

Sources NIGMS, 2025 · Burgess et al., 2003 · Baehr et al., 2000

An older tradition drew its own clock, 2,000 years earlier — compare them

Chapter 03

The waves of sleep

As you drift off, the brain slows down.

Four brain-wave rhythms, from awake to deep sleep Four stacked wave strips. Beta, awake and alert, is fast and low. Alpha, relaxed with eyes closed, is slower. Theta, light sleep, slower again. Delta, deep sleep, is slow and tall. BETA · AWAKE, ALERTALPHA · RELAXED, EYES CLOSEDTHETA · LIGHT SLEEPDELTA · DEEP SLEEP
  • 1924 Hans Berger records the first human EEGHaas, 2003
  • Delta slow, tall waves — the signature of deep "slow-wave" sleepNINDS, 2025

Chapter 04

What sleep gives back

The quiet work of a good night.

Sleep debt compounds, and hides from the person carrying it.

What a full night gives back

  1. Memory, consolidated
  2. Appetite and mood, regulated
  3. Physical repair, in the slow-wave hours

Sources NINDS, 2025

Chapter 05

The Eastern hour

The Hour of Rest · 9–11 PM

“Providence has given us hope and sleep as a compensation for the many cares of life.”

Voltaire

The full twelve-hour map, and waking at 3 AM

Chapter 06

Why we left melatonin out

It shifts timing. For everyday sleep, the effect is modest.

Our choice instead

  1. Calm the nervous system
  2. Ease the mind
  3. Lower body temperature a little
  4. No hormone — it won't shift your clock; it's for the hour you already keep

See every ingredient and dose

Sources
  1. NHLBI (2022) Sleep phases and stages
  2. NINDS (2025) Brain basics: Understanding sleep
  3. NIGMS (2025) Circadian rhythms
  4. Hilditch & McHill (2019) Nature and Science of Sleep
  5. Burgess et al. (2003) Behavioral Sleep Medicine
  6. Baehr et al. (2000) Journal of Sleep Research
  7. Haas (2003) J Neurol Neurosurg Psychiatry
  8. Watson et al. (2015) Sleep
  9. Ng et al. (2026) NCHS Data Brief 559
  10. Van Dongen et al. (2003) Sleep
  11. Ferracioli-Oda et al. (2013) PLoS ONE
  12. Sateia et al. (2017) J Clin Sleep Med

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