Sleep: Your circadian rhythm and how to work with it

Your body has a 24-hour internal clock that has been running since before you were born. Modern life is remarkably good at breaking it.

Every cell in your body contains a biological clock — a set of genes that cycle through activity and rest on a roughly 24-hour schedule. These internal clocks are coordinated by a master pacemaker in the brain called the suprachiasmatic nucleus (SCN), located in the hypothalamus. Together they constitute your circadian rhythm: a tightly orchestrated system that regulates sleep timing, hormone secretion, metabolism, immune function, body temperature, and cognitive performance.

Circadian biology won the Nobel Prize in Physiology or Medicine in 2017. It is not a wellness concept. It is fundamental physiology.

What your circadian rhythm actually controls

The circadian system does far more than determine when you feel sleepy. In the hours before your usual sleep time, your SCN triggers a rise in melatonin that begins the physiological preparation for sleep: core body temperature starts to drop, blood pressure falls, and alertness begins to wind down. This process — called the sleep pressure ramp — is sensitive to light, meal timing, physical activity, and temperature.

In the morning, your SCN initiates a cortisol pulse (the cortisol awakening response) that prepares you for wakefulness: alertness rises, blood pressure increases, and metabolic rate climbs. Timed correctly, this is your body doing exactly what it should. Disrupted or misaligned, it produces the chronic dysregulation associated with shift work, jet lag, and the increasingly common pattern of late-night screens followed by early alarms.¹

The practical consequence is this: it is not just how much you sleep that matters, but when you sleep relative to your circadian phase. Sleeping at the wrong time — even for adequate duration — produces impaired cognitive function, disrupted hormonal patterns, and measurable metabolic dysfunction.²

Chronotypes are real

Not everyone has the same circadian phase. Chronotype — whether you are naturally an early riser, a night owl, or somewhere in between — is substantially determined by genetics. Approximately 40–50% of chronotype variation is heritable.³

Early chronotypes (morning people) have a circadian phase that peaks earlier — their cortisol awakening response, peak alertness, and melatonin onset all occur earlier than the population average. Late chronotypes experience these shifts later. Neither is superior. The problems arise when people are forced to live against their chronotype — what researchers call "social jet lag."

Social jet lag — the mismatch between your biological clock and your social schedule — affects an estimated 70% of the working population. People who are forced to wake significantly earlier than their natural wake time on workdays effectively experience mild jet lag every Monday morning, and partially recover over the weekend only to repeat the disruption. This chronic misalignment is associated with elevated markers of cardiovascular risk, obesity, depression, and impaired cognitive performance.⁴

Light is the most powerful time-keeper

The primary signal that sets your circadian clock is light — specifically, short-wavelength (blue-spectrum) light detected by specialised photoreceptors in the retina called intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells are exquisitely sensitive to the light-dark cycle and transmit timing signals directly to the SCN.

Morning light exposure is the most powerful way to anchor your circadian clock. Bright outdoor light within 30–60 minutes of waking suppresses residual melatonin, advances your circadian phase, and sharpens the cortisol awakening response. This single habit has downstream benefits throughout the day: earlier peak alertness, faster sleep pressure build-up, and earlier melatonin onset in the evening.⁵

Evening light — particularly the blue-spectrum light emitted by phones, tablets, and LED screens — does the opposite. It signals to the SCN that it is still daytime, suppresses melatonin production, delays sleep onset, and compresses the early-night deep sleep window. The effect is measurable: evening smartphone use for 2 hours can delay melatonin onset by up to 1.5 hours.⁶

How to support your circadian rhythm

• Get bright outdoor light within 30–60 minutes of waking — even on overcast days, outdoor light (1,000–10,000 lux) far exceeds indoor lighting (200–500 lux) and provides the strongest circadian anchor available.
• Maintain a consistent wake time 7 days a week — your circadian system is anchored to your wake time more than your sleep time. Varying wake time by more than 60–90 minutes between weekdays and weekends creates measurable social jet lag.
• Dim indoor lighting after 8–9pm — your home lighting environment in the 2–3 hours before sleep significantly affects melatonin onset. Warm-spectrum lighting (red/amber) has a much smaller circadian impact than standard cool-white LED lighting.
• Use blue-light filters or screen dimmers in the evening — software filters (Night Shift, f.lux) reduce blue-spectrum emission from screens, though they are not a complete substitute for reducing screen use overall.
• Eat your main meals earlier in the day where possible — meal timing is a secondary time-keeper for peripheral clocks. Late eating delays the metabolic circadian rhythm and has been independently associated with weight gain and impaired glucose tolerance.⁷
• Keep naps short and early — a 10–20 minute nap before 3pm can improve afternoon alertness without disrupting your sleep pressure build-up. Naps after 3pm, or longer than 30 minutes, reduce sleep pressure and make it harder to fall asleep at night.
• Expose yourself to natural darkness in the evening — spend time outside in the evening where possible. Natural darkness, even at dusk, reinforces the circadian signal that sleep time is approaching.

The 100 Great Years perspective

Circadian biology is the framework that makes almost everything else about sleep make sense. Why timing matters as much as duration. Why alcohol disrupts sleep even when it helps you fall asleep faster. Why shift workers have elevated rates of cardiovascular disease, diabetes, and certain cancers. Why teenagers are not lazy — they have a later chronotype, and forcing them to attend school at 8am is biologically equivalent to asking adults to start work at 5am. Understanding your circadian rhythm is one of the highest-leverage insights in health, because it does not just affect sleep — it coordinates the biological machinery that determines how well every other system in your body functions. 100 Great Years tracks sleep because the circadian system is the conductor. Getting it right amplifies everything else.

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This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making decisions about your health.