Are you a lark who springs from bed at dawn, or a night owl happy staying up late? Your skin knows, according to a new study. By testing skin samples from people with sleep problems, researchers have found the first ties between people’s behavior and the biological clocks they carry in their cells. The discovery could lead to cheaper and more practical ways to diagnose and treat sleep disorders and other ailments.
To make the most of the day, humans and other animals evolved roughly 24-hour internal clocks known as circadian rhythms. A cluster of brain cells called the suprachiasmatic nucleus (SCN) runs the clock in humans, and the body's other cells have their own "slave clocks" that are synchronized to the SCN. With timekeeping outposts throughout the body, the main clock helps regulate everything from sleepiness to concentration. Buried in the brain, the SCN is not easy to study. So biochemist Steven Brown of the University of Zurich, Switzerland, and colleagues at the Free University of Berlin looked at slave clocks in skins cells, which are much more accessible.
They began by recruiting 28 extreme night owls and larks through newspaper and television ads. The team collected two small skin samples from each, and then cultured the cells in the lab and inserted the gene that makes fireflies glow into the gene that regulates the cells' clock. The result: luminous cells whose light waxes and wanes in time with their internal cycle, allowing the researchers to watch the clocks at work. To the scientists' surprise, the cells didn't desynchronize, and their behavior in the lab reflected the behavior of the humans they came from. The longest glowing cells belonged to night owls; lark skin cells glowed for shorter times, revealing a faster cycle.
"It's like a wristwatch: If it runs slow, you're late for everything; if it's too fast, you're early for everything," Brown explains. When the researchers induced the clock cycles to shift with chemicals or slight changes in temperature, they found that the cells also varied in their adaptability to change, in keeping with the volunteers' reports of varying responses to jet lag and other disruptions.
The study, published online this week in the Proceedings of the National Academy of Sciences, raises the possibility of an inexpensive and objective test of a person's "owlness" or "larkness." Such a test would be no small matter, given the prevalence of sleep disorders and the fact that many drugs, including cholesterol medications and chemotherapy, work more effectively if administered at certain points in a person's sleep/wake cycle. Pinpointing individual clock cycles could pave the way for personalized sleep and drug therapies, says Achim Kramer, a Free University chronobiologist who helped design the study.
Geneticist Paolo Sassone-Corsi of the University of California, Irvine, sees the study as an important and ingenious step. "No one had thought about taking biopsies to do this," he says. "This is a new tool, a possible new gateway to understand human rhythms."