BY late January many of us residing in northern latitudes aren’t sleeping well, overeat and are looking forward to the long sunlit days of July. Some people even get clinically depressed: a recent study revealed that some 10 percent of New Hampshire residents suffer from seasonal affective disorder. For too many people, this might seem like just a quirk of their personalities, or worse, a shortcoming. But the cause for our malaise lies in the working of our genes, organs — and, ultimately, in the chemical structure of moon rocks, like the ones returned by the Apollo space program.

Our perception of time defines the ways we interact with the planet and with one another. Humanity’s increasing need to communicate and trade has led to an ever-finer parsing of the moments of our lives with each passing year. Our need to segment a day into milliseconds — as with high-frequency stock trades — would probably have shocked our ancestors as much as a jet plane landing in the ancient African savanna.

But some clocks have not changed with technology, human interchange or commerce. Virtually every part of us — all our organs, tissues and cells — are set to a rhythm of day and night. Kidneys slow down at night. That’s a wonderful trait if you want to minimize trips outside of bed. The human liver works slowest in the morning hours, meaning the cheapest dates would be at breakfast.

How do these biological rhythms come about? We carry more than two trillion clocks inside of us. Our cellular clocks reside in the molecular machinery of DNA, which makes proteins that interact with one another and with DNA itself. Some combinations of these biological factors form a kind of molecular pendulum that swings back and forth between high and low levels of protein and gene activity, tuned to a virtual 24-hour day.