Obesity reviews : an official journal of the International Association for the Study of ObesityOct 24, 2009

How the body's internal day-night clocks in the brain and other organs are controlled in humans

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Mar 25, 2026

Abstract

The expression of clock genes has been investigated in various human tissues, including white blood cells and skin.

Circadian rhythms are regulated by central and peripheral clocks located in different brain regions and tissues.
Clock genes and their protein products play a crucial role in the functioning of these clocks.
Research has focused on comparing expressions of clock genes in human samples from several peripheral tissues.
Monitoring clock gene expression in peripheral tissues may provide insights into their regulation compared to the central clock.
Subjects undergoing a simulated night shift protocol showed changes in the clock of blood cells in response to bright light treatment.

Simplified

Many circadian rhythms are controlled by the central clock of the suprachiasmatic nucleus of the hypothalamus, as well as clocks located in other brain regions and most peripheral tissues. These central and peripheral clocks are based on clock genes and their protein products. In recent years, the expression of clock genes has started to be investigated in human samples, primarily white blood cells, but also skin, oral mucosa, colon cells, adipose tissue as well as post-mortem brain tissue. The expression of clock genes in those peripheral tissues offers a way to monitor human peripheral clocks and to compare their function and regulation with those of the central clock, which is followed by markers such as melatonin, cortisol and core body temperature. We have recently used such an approach to compare central and peripheral rhythms in subjects under different lighting conditions. In particular, we have monitored the entrainment of the clock of blood cells in subjects undergoing a simulated night shift protocol with bright light treatment, known to efficiently reset the central clock. This line of research will be helpful for learning more about the human circadian system and to find ways to alleviate health problems of shift workers, and other populations experiencing altered circadian rhythms.

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How the body's internal day-night clocks in the brain and other organs are controlled in humans

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