Sleep Science · Part 3 of 6
By Cambridge Sleep Sciences · Medical review: Dr Chris Dickson, 2 July 2026.
Before alarm clocks, before artificial light, before shift work, your body had already spent millions of years developing a timekeeping system more precise than most technology. The circadian rhythm is a roughly 24-hour biological clock that coordinates sleep, hormones, metabolism, immune function, and cognition. It's not just about feeling sleepy at night. It runs through virtually every cell in your body, and when it's working well, you probably don't notice it. When it isn't, nearly everything suffers.
How the clock actually works
The master clock lives in a tiny region of the hypothalamus called the suprachiasmatic nucleus (SCN), a cluster of around 20,000 neurons that receives direct input from light-sensitive cells in the retina. These cells, known as intrinsically photosensitive retinal ganglion cells, don't contribute to your vision. They exist specifically to detect ambient light levels and relay that information to the SCN, which uses it to calibrate the clock each day. This calibration process is called entrainment.
The SCN then synchronises peripheral clocks throughout the body, in the liver, gut, heart, lungs, and virtually every other tissue. These peripheral clocks are responsive not just to light signals from the brain, but to other time cues including meal timing, exercise, and temperature. They run in rough synchrony under normal conditions, but they can be pulled out of alignment by lifestyle factors. This is part of why irregular eating schedules, late-night exercise, or travelling across time zones can disrupt more than just sleep. 1
At a molecular level, the clock is driven by interlocking feedback loops of gene expression: proteins like CLOCK, BMAL1, PER, and CRY that activate and suppress each other in a cycle taking approximately 24 hours to complete. The 2017 Nobel Prize in Physiology or Medicine was awarded to the researchers who first described this mechanism.
Light is the strongest signal
Among all the cues that set and reset your circadian rhythm, light is dominant. Morning light, particularly the blue-wavelength light in natural daylight, suppresses residual melatonin and triggers a cortisol pulse that signals the start of the biological day. Without this cue, the internal clock tends to drift slightly later over time, which is why people who rarely see morning light often find their sleep progressively shifting toward later hours.
In the evening, the dynamic reverses. As light levels fall, the pineal gland begins secreting melatonin, not as a sedative, but as a darkness signal that coordinates the body's transition toward the physiological conditions suited to sleep. Melatonin secretion typically begins two to three hours before habitual sleep onset, peaks around the middle of the night, and suppresses again before waking. 2
This is why light exposure timing matters so much, and why modern lighting environments can work against the system. A 2015 study by researchers at Harvard and Brigham and Women's Hospital found that reading on a light-emitting device in the evening suppressed melatonin by over 55%, delayed sleep onset by an average of 10 minutes, and shifted the circadian phase by more than 1.5 hours compared with reading a printed book. 3
Chronotypes: not everyone's clock is set the same
Chronotype is the genetically influenced tendency toward being an early riser or a night owl. It isn't simply a preference. It reflects real differences in the timing of biological rhythms, including the phase of melatonin onset, the profile of core body temperature, and peak cognitive performance windows.
Chronotypes are partially heritable, change across the lifespan (teenagers genuinely shift later; older adults shift earlier), and are distributed across a wide spectrum rather than two neat camps. What matters practically is that your chronotype tells you something about when your physiology is primed for sleep. Forcing sleep significantly out of alignment with that window makes sleep harder and less restorative. 4
When social schedules force people with late chronotypes to wake significantly earlier than their biology prefers, a phenomenon researchers call social jetlag, the consequences extend beyond tiredness. A 2015 study in the Journal of Clinical Endocrinology and Metabolism found that social jetlag was independently associated with higher adiposity and adverse metabolic markers, even after controlling for total sleep duration. 5
What disrupts the clock
Artificial light at night. The most pervasive circadian disruptor in modern life. Evening exposure to blue-wavelength light delays melatonin onset and shifts the clock later, making it harder to fall asleep at a consistent time and harder to wake refreshed.
Irregular sleep timing. The circadian system is partly entrained by the regularity of its anchoring cues, particularly wake time and morning light. Going to bed and waking at highly variable times reduces the strength of the clock's signal and degrades sleep quality, even when total sleep time is adequate.
Meal timing. The liver clock and gut clocks are entrained partly by when food arrives. Late-night eating sends time signals to peripheral clocks that conflict with the brain's light-driven cues, creating internal desynchrony. Research links this misalignment to impaired glucose metabolism and metabolic dysfunction independently of caloric intake. 6
Shift work. Chronic misalignment between the internal clock and external schedule, as experienced by shift workers and frequent long-haul travellers, is associated with significantly elevated risk of metabolic syndrome, cardiovascular disease, and certain cancers. This is not purely about sleep loss. It reflects the broader physiological cost of running all the body's clocks out of synchrony.
What you can do
Get morning light. Ten to thirty minutes of outdoor light within an hour of waking is one of the most effective ways to anchor the circadian clock. On overcast days, natural light still far exceeds typical indoor lighting levels.
Protect evening darkness. Dim lights after 8 or 9pm, use warmer-spectrum bulbs, and if you use screens in the evening, reduce brightness and use a blue-light filter. Not because the filter is transformative on its own, but because it reduces the suppressive effect on melatonin.
Eat earlier. Finishing the last substantial meal at least two to three hours before sleep aligns meal timing more closely with the peripheral clock's preferences and reduces the circadian conflict that comes from eating late.
Prioritise consistency. A regular wake time is more powerful than a regular bedtime. It anchors the clock even on nights when sleep is delayed.
Takeaways
The circadian rhythm is a precise 24-hour biological clock running in the brain and every major organ, coordinated by the hypothalamus in response to light. Melatonin is the darkness signal, not a sleep drug. It tells the body the biological day is ending and sets the stage for the sleep that follows. Evening light exposure suppresses melatonin and delays the clock; morning light entrains and anchors it. Chronotype is partly genetic and reflects genuine differences in biological timing. Forcing people significantly against their chronotype is associated with measurable health consequences. Consistent sleep timing, morning light, and eating earlier in the day are the most evidence-supported levers for circadian health.
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References
- Abbott SM, et al. Circadian rhythm sleep-wake disorders. Psychiatr Clin North Am. 2015;38(4):805-23.
- Cajochen C, et al. Role of melatonin in the regulation of human circadian rhythms and sleep. J Neuroendocrinol. 2003;15(4):432-7.
- Chang AM, et al. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. Proc Natl Acad Sci USA. 2015;112(4):1232-7.
- Montaruli A, et al. Biological rhythm and chronotype: new perspectives in health. Biomolecules. 2021;11(4).
- Wong PM, et al. Social jetlag, chronotype, and cardiometabolic risk. J Clin Endocrinol Metab. 2015;100(12):4612-20.
- St-Onge MP, et al. Effects of diet on sleep quality. Adv Nutr. 2016;7(5):938-49.
