A light touch changes the biological clock.

A light touch changes the biological clock

Airline passengers leaving San Francisco for London might one day use a precisely timed dose of light to reset their biological clocks to English time and avoid debilitating jet lag, if future research confirms a study reported this week.

Scientists have just begun unraveling the complicated circadian rhythm that governs sleepiness, hormone levels and other daily cycles of the body. Three years ago, researchers in Boston described a case study in which a 66-year-old woman with a fast-running biological clock sat in front of bright lights each night before going to bed (SN: 8/30/86, p. 136). Just one exposure to light shifted many aspects of her circadian cycle -- a finding that surprised scientists, who believed the human biological clock was not very sensitive to light but instead relied on social cues.

The same research team now has completed a larger study designed to find out how people with normal sleep patterns react to different light patterns given at varying stages in their circadian cycles. Charles A. Czeisler, director of the Center for Circadian and Sleep Disorders Medicine at Brigham and Women's Hospital in Boston, and his colleagues studied 14 men aged 18 to 24. The subjects checked into the Boston sleep laboratory, where scientists charted their normal biological cycles using hormone levels and endogenous body temperature, a measure that corrects for subtle temperature fluctuations caused by activity. Subjects then went through three 24-hour cycles that included eight hours of darkness, 11 hours of indoor room light and five hours of bright light equivalent to sunlight.

In 45 trials, the researchers found that the men with the largest shifts in their biological clocks had been exposed to light at a time when their endogenous body temperatures were at a minimum, generally about two to three hours before their normal waking time. The team reports in the June 16 SCIENCE that subjects exposed to light at that time experience a shift of up to 12 hours in their biological clocks--a complete inversion of the cycle, equivalent to a New Yorker getting accustomed to Bangkok time.

The researchers found they could reset the biological clock backward or forward by giving certain amounts of light at various points in the circadian cycle. In addition, they discovered that artificial room light affects both the direction and the magnitude of a shift induced by exposure to bright light. That finding may be important for insomniacs who turn on a light to read, Czeisler says.

Eventually, such research may lead to light therapy for people with certain types of sleep disorders, such as elderly people who wake up in the predawn hours because of a fast-running clock, Czeisler says. But, he cautions, "this is basic research. It's really too early to give a prescription."

Sleep researcher Charmane Eastman at the Rush Presbyterian Saint Luke's Medical Center in Chicago agrees, adding that research conducted in a laboratory may not reflect the real world. More work needs to be done to find practical ways for people to alter their biological clocks, she says.