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Circadian Rhythm

Jan 01, 20243 min read

Circadian Rhythm

Your body runs on a 24-hour clock. Fight it, and everything breaks—sleep, metabolism, mood, cognition. Work with it, and biology does the heavy lifting.

What It Controls

The circadian rhythm regulates:

  • Sleep-wake cycles
  • Hormone release (cortisol, melatonin, growth hormone)
  • Body temperature
  • Metabolism
  • Cognitive performance
  • Immune function

This isn’t optional. Your biology expects light in the morning, food during the day, and darkness at night.

The Master Clock

Suprachiasmatic Nucleus (SCN)

The SCN is a cluster of ~20,000 neurons in your hypothalamus (Czeisler et al., 1999). It:

  • Receives light input directly from your eyes
  • Synchronizes clocks in every organ
  • Times hormone release

Your liver, gut, and muscles all have their own clocks. The SCN keeps them in sync.

Zeitgebers (Time Givers)

External cues that set your clock:

ZeitgeberEffectTiming
LightMost powerful; sets the clockMorning light advances, evening light delays
FoodSets peripheral clocks (liver, gut) (Panda, 2016)Eating within a consistent window
ExerciseModerate phase-shifting effectMorning exercise advances clock
TemperatureBody temp follows circadian patternCool environment for sleep
Social cuesWeak but present effectRegular schedules help

Light is King

  • Morning bright light (10,000+ lux) advances the clock → earlier sleepiness
  • Evening blue light delays the clock → later sleepiness
  • The SCN is most sensitive to blue wavelengths (~480nm)

The Daily Rhythm

Morning (6am-12pm)

  • Cortisol peaks (~30 min after waking) — “cortisol awakening response”
  • Body temperature rises
  • Alertness and cognitive performance increase
  • Best time for: focused work, exercise, difficult tasks

Afternoon (12pm-6pm)

  • Reaction time and coordination peak (~2-4pm)
  • Brief alertness dip post-lunch (not just food—it’s circadian)
  • Best time for: physical performance, collaborative work

Evening (6pm-10pm)

  • Melatonin begins rising (~2hr before habitual bedtime)
  • Body temperature begins dropping
  • Best time for: winding down, creative tasks, light activity

Night (10pm-6am)

  • Melatonin peaks
  • Growth hormone released in early sleep
  • Memory consolidation during sleep
  • Lowest body temperature ~4am

Circadian Disruption

Consequences of Misalignment

Chronic circadian disruption is linked to:

  • Metabolic dysfunction: Increased diabetes risk, weight gain
  • Cardiovascular disease: Higher blood pressure, heart disease risk
  • Cognitive impairment: Reduced memory consolidation, attention
  • Mood disorders: Increased depression and anxiety risk
  • Immune suppression: More susceptible to illness
  • Cancer risk: Night shift work classified as “probable carcinogen” (International Agency for Research on Cancer, 2019)

Common Disruptors

  • Social jet lag: Different sleep schedules on weekdays vs weekends (Roenneberg et al., 2012)
  • Shift work: Working against the natural light-dark cycle
  • Late-night screen use: Blue light suppresses melatonin
  • Irregular meal timing: Confuses peripheral clocks
  • Travel across time zones: Classic jet lag

Optimizing Your Circadian Rhythm

Morning Protocol

  1. Wake at consistent time (±30 min, including weekends)
  2. Get bright light immediately (sunlight or 10,000 lux light box)
  3. Delay caffeine 90 min (let cortisol do its job first)
  4. Exercise if possible (amplifies morning alertness)

Evening Protocol

  1. Dim lights after sunset (or use blue-blocking glasses)
  2. Stop eating 2-3 hours before bed (let digestion complete)
  3. Reduce screen brightness (use night mode/f.lux)
  4. Keep bedroom dark and cool (65-68°F / 18-20°C)

Consistency is Everything

The single most important intervention: fixed wake time. Your wake time anchors the entire circadian system. Variable wake times create chronic mini-jet-lag.

Related Content

Protocols:

Concepts:

Key Insight

Your body doesn’t just need sleep—it needs sleep at the right time.

8 hours from 3am-11am ≠ 8 hours from 10pm-6am. Alignment matters as much as duration (Walker, 2017).

Czeisler, C. A., Duffy, J. F., Shanahan, T. L., Brown, E. N., Mitchell, J. F., Rimmer, D. W., Ronda, J. M., Silva, E. J., Allan, J. S., Emens, J. S., Dijk, D.-J., & Kronauer, R. E. (1999). Stability, Precision, and Near-24-Hour Period of the Human Circadian Pacemaker. Science, 284(5423), 2177–2181. https://doi.org/10.1126/science.284.5423.2177
International Agency for Research on Cancer. (2019). Night Shift Work. IARC Monographs on the Identification of Carcinogenic Hazards to Humans.
Panda, S. (2016). Circadian physiology of metabolism. Science, 354(6315), 1008–1015. https://doi.org/10.1126/science.aah4967
Roenneberg, T., Allebrandt, K. V., Merrow, M., & Vetter, C. (2012). Social Jetlag and Obesity. Current Biology, 22(10), 939–943. https://doi.org/10.1016/j.cub.2012.03.038
Walker, M. (2017). Why We Sleep: Unlocking the Power of Sleep and Dreams. Scribner.

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