Almost all of the body’s functions, sleep and wakefulness above all, follow a cycle of about 24 hours. Hormone release, body temperature, blood pressure and metabolism follow the same cycle. This rhythm is called the circadian rhythm. The word comes from the Latin circa, meaning “about”, and dies, meaning “day”.
Our master clock is in the brain, in the hypothalamus. A structure called the suprachiasmatic nucleus lies just above the point where the optic nerves cross. It receives light information directly from the eyes. In this way it resets our internal clock every day according to sunrise and sunset. At the cellular level, the clock works through a few genes switching each other on and off. This cycle takes about 24 hours to complete. Organs such as the liver, pancreas and gut also have their own clocks. Good health requires these clocks to work in harmony.
Melatonin is one of the most important elements of this system. It is secreted by the pineal gland. Its level starts to rise 2-3 hours before habitual bedtime. Evening light suppresses melatonin release and delays the internal clock. This is one reason why looking at screens late into the night keeps us awake.
Not everyone’s body clock is set the same way. Some people are morning types, others are night owls. This is called chronotype. In adolescence the internal clock naturally shifts to later hours. Teenagers’ difficulty getting up in the morning is largely biological. In some people this shift is very marked. They cannot fall asleep before 3 or 4 a.m. and cannot wake before noon. This is called delayed sleep-wake phase disorder. In some families, this condition has been linked to a change in one of the clock genes. The opposite also exists. In advanced sleep phase, the person falls asleep early in the evening and wakes very early in the morning. Having a different chronotype is not a disease. It becomes a disorder only when the shift causes problems in the person’s life.
Daily life may not fit the internal clock. Getting up early on weekdays and sleeping until noon at weekends is one example. Researchers call this social jet lag. Shift work is another such mismatch. A person working at night is exposed to light, moves and eats at times when the body is preparing for rest and fasting. They try to sleep during the day, but their sleep is broken. Even among those on permanent night shifts, most never fully adapt. Shift work has been associated with weight gain and an increased risk of cardiovascular and metabolic disease. The International Agency for Research on Cancer, part of the World Health Organization, has classified night shift work as a probable carcinogen.
Putting the clocks forward creates a similar problem. The clock changes but the sun does not. Our internal clock keeps following the sun. For this reason, most people never fully adapt biologically to the change. A recent study showed that migraine attacks increased after the switch to daylight saving time, even though sleep duration did not change.
The circadian rhythm is not only about sleep. It is intertwined with many neurological diseases. Stroke most often begins between 6 and 9 a.m. During these hours, factors that promote clotting increase and blood pressure rises. Cluster headache often starts around 2 a.m. and peaks in spring and autumn. Migraine, by contrast, tends to begin in the late morning and around midday.
In Alzheimer’s and Parkinson’s disease, disruption of the internal clock may be one of the early signs. In healthy older women monitored with wrist-worn movement sensors, a weakening of the daily rest-activity rhythm has been shown to increase the risk of mild cognitive impairment and dementia in later years. A 2020 study following about 3000 older men found the same association for Parkinson’s disease. Once the disease has developed, the disrupted rhythm may contribute to its progression.
Hospitals are among the worst places for the internal clock. Patients are woken day and night. Daytime room light usually does not exceed 150 lux. This is far below outdoor daylight.
Light is the strongest signal that sets the internal clock. Light received in the biological morning moves the clock earlier. Evening light moves it later. Melatonin, on the other hand, moves the clock earlier when taken in the evening and later when taken in the morning. When it is used to reset the internal clock, low doses are recommended. Slow-release products may keep acting at unwanted times. Meal times also set the clocks in the organs. In animal experiments, feeding only during the rest period uncoupled the organ clocks from the clock in the brain. Feeding only during the active hours, in contrast, has been associated with weight loss and better blood sugar control.
References
Abbott SM. Circadian neurology. Continuum (Minneap Minn). 2026;32(4):1184-1200.
Gorantla S, Velaga A, Ravisankar A, Nersesyan H, Sundar KM, Johnson KG. The effect of standard time to daylight-saving clock transition on sleep and migraine headaches. J Clin Sleep Med. 2026;22(1):18. doi:10.1007/s44470-025-00006-2
Tranah GJ, Blackwell T, Stone KL, et al. Circadian activity rhythms and risk of incident dementia and mild cognitive impairment in older women. Ann Neurol. 2011;70(5):722-732. doi:10.1002/ana.22468
Leng Y, Blackwell T, Cawthon PM, et al. Association of circadian abnormalities in older adults with an increased risk of developing Parkinson disease. JAMA Neurol. 2020;77(10):1270-1278. doi:10.1001/jamaneurol.2020.1623
Damiola F, Le Minh N, Preitner N, et al. Restricted feeding uncouples circadian oscillators in peripheral tissues from the central pacemaker in the suprachiasmatic nucleus. Genes Dev. 2000;14(23):2950-2961. doi:10.1101/gad.183500
Regmi P, Heilbronn LK. Time-restricted eating: benefits, mechanisms, and challenges in translation. iScience. 2020;23(6):101161. doi:10.1016/j.isci.2020.101161
