Tracking your internal clock may be as easy as plucking a few strands of hair, according to a new study.

The research, published today in the journal Proceedings of the National Academy of Sciences, found that hair follicles hold a record of the gene activity that influences when we wake and when we sleep. The results could be used to diagnose and study sleep disorders and conditions like jet lag.

Whether you're a night owl or a morning lark, your sleep-wake cycle is controlled in large part by genes called clock genes. These genes vary their activity throughout the day, setting the internal clock that drives our circadian rhythms.

The first human clock gene was discovered almost 10 years ago, but isolating the genes efficiently enough to study sleep-wake cycles in real time has proved difficult. When the genes are active, they transcribe their DNA into RNA, the first step in producing various proteins that essentially carry out a gene's instructions and, in this case, influence circadian rhythms. The RNA can be found in cells all over the body, from white blood cells to the lining of the mouth, but techniques for extracting it from these cells proved unreliable.

So Makoto Akashi, a researcher at Yamaguchi University in Japan, and colleagues turned to hair. At the base of every strand of hair is a follicle of living cells, which clings to the hair when plucked. By tweezing an average of 10 head hairs per person (five for thick-haired folks and as many as 20 for those with thin locks), the researchers were able to isolate and track the activity of three separate clock genes. Beard hair was even more reliable: Just three strands produced accurate results.

By plucking hair from four people going about their daily routines, the researchers found that peak gene expression (when the most transcribed RNA was present) matched peak wakefulness. When people shifted their schedules by four hours over three weeks, however, peak gene activity shifted by just 2.1 hours, suggesting a jet lag-like reaction. In other words, even though people forced themselves into a new sleep-wake schedule, their body clocks had yet to adjust.

The researchers then followed six shift workers whose schedules alternated between 6 a.m. to 3 p.m. one week and 3 p.m. to midnight the next. They found that even though the workers' sleep-wake cycles shifted by seven hours between weeks, their gene expression shifted by just 2 hours. That could mean that shift workers live in a permanent state of jet lag, possibly putting them at risk for heart attack and stroke, the researchers wrote.

For now, the hair-plucking technique is best suited for research purposes, but with technical improvements, the researchers say, it could be used in hospitals and doctors' offices to diagnose and treat patients with skewed circadian rhythms.