The master clock
The circadian clock is centered in the suprachiasmatic nucleus (SCN) of the hypothalamus, a small bilateral structure receiving direct input from the retina. The SCN coordinates peripheral clocks present in nearly every cell of the body. Together they regulate:
- Hormone release timing
- Body temperature
- Sleep-wake cycle
- Immune function
- Metabolism
- Gene expression rhythms
Entrainment
The clock has a free-running period of slightly longer than 24 hours but is entrained (synchronized) primarily by:
- Light exposure (especially morning blue-spectrum)
- Eating timing
- Activity timing
- Temperature changes
Hormonal rhythms
Major rhythms (covered in detail in the diurnal rhythms article):
- Cortisol: peaks 30-45 min post-wake
- Testosterone: peaks at waking (men)
- Growth hormone: 4-5 pulses during deep sleep
- Melatonin: rises with darkness, peaks during sleep
- Thyroid: modest evening peak
- Insulin sensitivity: best in morning
Modern disruptors
- Artificial light at night (especially blue-spectrum)
- Phone/screen use late evening
- Inconsistent sleep timing
- Shift work
- Jet lag (frequent flyers)
- Late-night eating
- Late caffeine
- Late alcohol (degrades sleep architecture)
- Reduced morning light exposure (indoor work)
Effects of disruption
Circadian disruption produces:
- Suppressed morning cortisol rise
- Suppressed nocturnal GH
- Reduced testosterone (in men)
- Reduced melatonin
- Impaired insulin sensitivity
- Increased inflammation
- Mood and cognitive effects
- Increased cancer risk (shift workers)
Aligning circadian biology
- Morning sunlight exposure (10+ minutes within first hour of waking)
- Consistent sleep-wake times (even weekends)
- Limit late blue light (sunset onward, dim/red lighting)
- Earlier eating window (last meal 3+ hours before sleep)
- Limit late caffeine (cutoff 12 hours before sleep)
- Limit alcohol (especially close to bedtime)
- Cool, dark sleeping environment
- Regular exercise timing
The clinical insight: Circadian alignment is one of the most underused health interventions. The fixes are largely free and behavioral. The benefits, better sleep, better hormones, better metabolism, better mood, compound across all systems.
Bottom line
The circadian clock coordinates hormone release, metabolism, sleep, and immunity. Modern life systematically disrupts it. Aligning circadian inputs (morning light, consistent timing, early eating, dim evenings) supports system-wide health. The intervention is largely free and behavioral but profoundly powerful.