Every cell in the body contains a molecular clock, a set of interlocking protein feedback loops that oscillate on a 24-hour cycle and regulate the timing of cellular processes from DNA repair to protein synthesis to inflammatory signalling. These cellular clocks are synchronised to the external light-dark cycle by the master clock in the suprachiasmatic nucleus of the hypothalamus. When the internal clock is chronically misaligned with the external environment, as in shift workers, frequent international travellers, and people with highly irregular sleep schedules, the downstream consequences are pervasive.
The Epidemiology of Shift Work
Shift workers who work at night and sleep during the day have consistently elevated rates of metabolic syndrome, type 2 diabetes, cardiovascular disease, certain cancers, and accelerated cognitive decline. The International Agency for Research on Cancer has classified night shift work as a probable human carcinogen. These outcomes are not explained by other lifestyle factors: they persist after controlling for sleep duration, smoking, obesity, and physical activity. The mechanism is circadian misalignment itself.
Light and the Circadian Clock
Light is the primary environmental synchroniser of the circadian clock. Exposure to blue-wavelength light in the evening suppresses melatonin secretion and delays the circadian phase, pushing the internal body clock later. This is the mechanism by which screens and artificial lighting in the evening fragment sleep and produce the social jet lag that characterises modern living. Morning bright light exposure, ideally within 30 to 60 minutes of waking, is the most powerful circadian entrainment signal available without pharmacological intervention.
Sources
- Czeisler, C.A., et al. (1999). Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 284(5423), 2177-2181.
- Scheer, F.A.J.L., et al. (2009). Adverse metabolic and cardiovascular consequences of circadian misalignment. Proceedings of the National Academy of Sciences, 106(11), 4453-4458.
- IARC Working Group on the Evaluation of Carcinogenic Risk to Humans. (2010). Painting, firefighting, and shiftwork. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, 98.