The Quest by Circadian Medicine to Make the Most of Our Body Clocks
7th July 2022
Many of us are passingly familiar with circadian rhythms as a way to refer to our sleep cycle. In 1972, scientists discovered that that cycle is mediated by an area in the brain’s hypothalamus called the suprachiasmatic nucleus. This structure coordinates the release of hormones — among them dopamine — that lower body temperature and blood pressure and make us feel sleepy; in the morning, cortisol and other hormones restore our alertness, make us warmer and increase blood pressure. The a.m. surge in blood pressure is believed to be one reason heart attacks occur more often then than in the p.m.
In the past two decades, however, researchers have discovered that the clock in the brain is by no means the only one in our body. It turns out that most of our cells contain a group of genes that might be thought of as gears in a mechanical watch, keeping time everywhere internally. These “clock genes” — there are at least six that are considered integral to the watch’s operation — work together the same way in each cell. And just as they cause the release of hormones in the brain, they dictate other processes in other parts of the body. In the early 2000s, advances in the ability to detect the activity of genes in various tissues revealed that the cell clocks are organized into separate organ-level clocks representing every physiological system: There’s a skin clock and a liver clock and an immune-system clock; there’s a clock for the kidney, heart, lungs, muscles and reproductive system. Each of those clocks syncs itself to the central clock in the brain like an orchestra section following its conductor. But those sections also adjust how and when they perform based on guidance they receive both from the environment and from one another, and their timing can provide feedback to the central clock and cause it to adjust the time it keeps too. The liver, for instance, determines when to rev up your metabolism based on when you eat; if you do that in the middle of the night, the liver will be receiving contradictory cues from the brain, which is telling it to rest. As a result, when the liver starts processing the midnight food, it will do so less efficiently than it would have done after a daytime meal — and it sends conflicting signals back to the brain and other organ systems.