For a while now, researchers have understood that human beings are equipped with built-in “clock genes” that exist in every cell in the body, and that are active or inactive at regular intervals throughout the day. These clock genes, in conjunction with environmental cues such as daytime sun exposure, help set the body’s internal circadian rhythms or schedules, which regulate a person’s appetite, hormone levels, body temperature, energy, sleep schedule, and much more.

“The reason for this is our planet is ruled by the solar system which generates a rhythmic environment, and over millions of years organisms have developed these clocks to cope with the environmental rhythms,” says Katharina Wulff, a chronobiologist and senior lecturer in molecular biology at Umeå University in Sweden.

But Wulff says the body’s internal clocks do not exactly overlap with the external, 24-hour clock set by our solar system. And this is by design. “This allows us the flexibility to adapt or adjust to different environments, such as seasonal shifts in daylight schedules,” she explains.

Most people have a great deal of overlap between their internal clocks and the external world’s 24-hour clock. They may run slightly ahead or behind, and so their preference for rising on the early side or staying up a bit later isn’t too dramatic. But in some — up to 10% of the population, Wulff estimates — there’s a sizeable discrepancy. This discrepancy creates an extreme preference for morningness or eveningness, she says. And the underlying genetic variations that account for this extreme preference for eveningness seem to be associated with a greater risk for health issues.

But why? One possibility is that the clock genes that are associated with night owl status also increase a person’s risk for various health issues. But another possibility is that work or school or social demands are more likely to impose unnatural sleep-wake schedules on evening types, as opposed to morning people. The resulting disconnect between when a night owl’s body wants to sleep and wake, and when life forces them to sleep and wake, may disrupt their natural internal rhythms in ways that promote poor sleep, unhealthy hormone shifts, metabolic issues, or other biological changes that could lead to things like cancer or depression.

“If people are always having to adhere to a schedule they’re not suited to, that may create health problems,” Wulff says. “The muscles and microbiome and eating and the heart — everything will be affected.” Wulff says all of this only applies to the small percentage of night owls who are at the extreme end of the spectrum. If you’re like most people — meaning you tend to get up between 5 and 9 a.m. and go to bed between 10 p.m. and one in the morning — you probably have little to worry about.