What actually is the circadian rhythm?

The body's internal ~24-hour clock, regulated primarily by the suprachiasmatic nucleus in the hypothalamus. It governs sleep / wake, hormone release, body temperature, digestion, alertness, and many other systems. It runs slightly longer than 24 hours and gets re-entrained each day by external cues — primarily light.

Light is the dominant signal, by an order of magnitude. Food timing, social cues, exercise, and temperature also contribute, but light at the right times is the high-leverage lever.

How is jet lag actually working?

Your internal clock has been moved several hours from the local sun. Until light re-entrains it, your sleep, hormone, and digestive systems are running on the old timezone. Westward travel is usually easier because your clock can be lengthened more easily than shortened; eastward travel takes about a day per hour to adjust.

Does shift work damage health?

Long-term shift work is associated with measurable increases in cardiovascular disease, metabolic dysfunction, and some cancers in observational studies — moderate-to-strong evidence. Mitigation strategies exist (consistent shift patterns, deliberate light management) but don't fully eliminate the cost. We'd label this as moderate evidence with non-trivial confounding.

Are blue-light blocking glasses worth it?

Modest effect at best. The mechanism is real (blue light suppresses melatonin) but the doses required for effect are larger than most evening exposures. Dimming lights, avoiding bright overheads, and shortening late-evening device time matter more. We'd label glasses as ‘emerging’ under our evidence policy.

What about teenagers — are they really wired differently?

Yes — teenagers' circadian rhythms shift later in adolescence, biologically, not behaviourally. Early school starts collide with this shift, with measurable cost to sleep and academic performance. The evidence here is strong.

Circadian rhythm

How the body clock works

The suprachiasmatic nucleus is a small cluster of cells in the hypothalamus that runs an oscillating clock with a period of slightly more than 24 hours. It receives input from light-sensitive cells in the retina and outputs signals that cascade across the body — telling the pineal gland when to release melatonin, the adrenals when to release cortisol, the digestive system when to ramp up, the body temperature regulator when to drop core temperature.

Without daily external cues, the clock would drift slightly later each day, which is why isolation studies (cavers, submariners, polar researchers) show progressive desynchronisation. The cues that re-entrain it are mostly free: light at the right times, consistent meal timing, predictable activity patterns, evening cool.

Light — the dominant signal

Light is the single most powerful entrainer of the clock. Two interventions account for most of the available improvement:

Outdoor morning light, 10-15 minutes, within 30 minutes of waking. Indoor light is dim by comparison (200-500 lux vs 10,000+ lux outside, even on cloudy days). The morning light pulse anchors the clock and starts the day's cortisol rise.
Dim evening light. The hour before bed should be the dimmest hour of your day. Bright overheads, screens at full brightness, harsh task lighting — all suppress melatonin and delay sleep onset.

This is mechanistically well-established and labelled ‘strong evidence’ under our policy.

Food, movement, temperature

Light dominates, but other cues matter:

Food timing. Eating creates a secondary signal that synchronises peripheral clocks (liver, gut). Consistent meal timing and an overnight fast support the rhythm; chaotic eating disrupts it.
Exercise timing. Morning exercise reinforces the morning cortisol rise. Late-evening intense exercise can delay sleep onset; gentle evening movement (walks) generally helps.
Temperature. Core body temperature falls in the hours before sleep; a cool bedroom (16-19°C / 60-67°F) supports the drop. Hot rooms work against the rhythm.

A daily protocol that works for most adults

The unglamorous template (most of this lives in the circadian rhythm checklist):

Fixed wake time within ±30 minutes seven days a week.
10-15 minutes of outdoor light within 30 minutes of waking.
First meal 1-2 hours after waking; consistent meal timing across the day.
Last caffeine 8+ hours before bed.
Last meal 2-3 hours before bed.
Dim lights and lower stimulation in the last hour before bed.
Cool, dark bedroom; phone outside the room.
Same routine on weekends (the easiest one to skip; the biggest leak).

When the clock is broken — shift work, jet lag

Shift work and frequent timezone changes are known stressors on the system. Mitigation strategies, in rough order of importance:

Consistent shift patterns where possible (better than rotating).
Aggressive light management — bright light on the ‘active’ phase, darkness on the ‘sleep’ phase.
Small-dose melatonin (0.3-0.5 mg) for circadian shifts — much smaller than the multi-mg sleep-aid doses sold over the counter. Talk to a clinician.
For jet lag: prepare 2-3 days in advance by shifting bedtime; expose to morning light on arrival.

Common mistakes

Optimising supplements while morning light exposure is zero.
Sleeping in on weekends and re-creating mild jet lag every Monday.
Strong evening exercise within 2 hours of bed.
Bright overheads in the bathroom right before sleep.
Eating heavy meals close to bedtime.
Treating circadian rhythm as ‘just for biohackers’ — it's the foundation everything else builds on.

Topic: Sleep better.
Topic: Recovery and energy.
Micro-course: The Sleep and Circadian Edge.
Worksheet: Circadian rhythm checklist.
Tool: Sleep consistency tracker.
Path: Stronger Body, Clearer Mind.