circadian rhythm

The National Institute of General Medical Sciences defines circadian rhythms as some of the physical, mental, and behavioral changes an organism experiences over a 24-hour cycle. They are produced by biological clocks, sets of proteins encoded by genes that switch on and off in sequence throughout the body. In humans the master clock exists in the brain, in a large group of nerve cells that form a structure called the suprachiasmatic nucleus, and its job is to keep every other clock in the body running in step with the rest.

Light is the main signal it reads. The institute states that light and dark have the biggest influence on circadian rhythms, and the master clock receives that information through the eyes. In the evening, a person's master clock tells their brain to make more melatonin, causing sleepiness, which is how the sleep and wake cycle is tied to the light and dark cycle outside. Because the pathway runs through the eyes rather than through anything a person decides, light reaching them at night carries the signal whether or not it was noticed.

The term to watch in research papers is circadian misalignment, meaning the internal cycle and the external one have come out of step. It is what shift work, jet travel and light at night are all proposed to cause, and it is the mechanism most often named when such exposures are linked to outcomes elsewhere in the body. Naming a mechanism is not the same as demonstrating one: in an observational study the circadian explanation is the authors' account of why a pattern might exist, offered alongside the pattern rather than established by it.