Early bird or late riser? The mysteries of your sleep cycle may be unlocked by the hairs on your head, a new study says.

That's because the genes that regulate our body clocks can be found in hair-follicle cells, researchers have discovered.

A tiny portion of the brain called the suprachiasmatic nucleus controls the human body clock, and RNA strands—protein-building chains of molecules—process these signals throughout the body in 24-hour cycles. (Get a genetics overview.)

Predicting Morning People

RNA strands containing the clock genes are found throughout the body—including in white blood cells and the inside of the mouth—but human hair is easiest for scientists to test.

So Makoto Akashi, of the Research Institute for Time Studies at Yamaguchi University in Japan, and colleagues pulled head and beard hairs from four test subjects at three-hour intervals for a full day. The subjects had already reported their preferred schedules for waking up and eating, among other lifestyle choices. (Take National Geographic magazine's sleep quiz.)

The test day occurred after the subjects had rigorously adhered to their preferred schedules for nine days—in other words, the morning people woke up early every day, and the late sleepers woke up late every day.

When the researchers tested the genes in the subjects' follicles, they found that body-clock gene activity peaked right after a subject had woken up, regardless of whether it was 6 a.m. or 10 a.m.

This suggests that the brain "turns on" the genes at different times of the morning in different people.

Other clock genes followed similar patterns, making it possible to predict "morning people" with just a pluck, the study said.

"Clock Gene" Tests to Give Health Warnings?

While most people may already know if they prefer to sleep in or wake up early, the new research might also provide insights into human health, researchers say. (See a human-body interactive.)

Disorders of the body clock have been implicated in high blood pressure, diabetes—even cancer. (Read about why we sleep in National Geographic magazine.)

The researchers also studied the hairs of rotating shift workers, who are at greater risk for body-clock disorders, for three weeks. Over that amount of time, the workers alternated from an early work shift (6:00 a.m. to 3:00 p.m.) to a late shift (3:00 p.m. to midnight).

But the three-week period wasn't enough time for the workers' internal clocks to readjust, according to measurements of follicle genes.

Even though the workers' lifestyle was shifted by seven hours, the clock-gene activity in their follicles shifted by only two hours—suggesting shift workers live in a state of jet lag, the study said.

The follicle test could be used to develop "working conditions that do not disturb clock function" by building in enough time to adjust, the authors wrote.

A noninvasive check for a clock disorder could serve as an early warning system, Akashi said: "I hope that our method will be used for regular health checks in schools and companies to keep healthy clocks."