Got no rhythm: stalling biological clocks.

Got no rhythm: Stalling biological clocks 

Critically timed exposures to bright light can virtually shut off the human "biological clock" temporarily, according to a study reported this week. The finding, which confirms previous investigations with plants and insects, aids the development of mathematical models of the complex circadian rhythm that governs sleepiness, hormone levels, body temperature and other daily biological cycles, says Charles A. Czeisler of Brigham and Women's Hospital in Boston.

Mathematical models of circadian peaks and valleys in biological functions may also increase the precision of timed-light treatments for jet-lagged travelers and weary shift workers, Czeisler adds.

In 1989, his team found that five-hour-long exposures to bright light equivalent to sunlight, administered on three consecutive days when body temperature reached its nadir (around two to three hours before waking), shifted volunteers' biological clocks by up to 12 hours, inverting the daily cycle (SN: 6/17/89, p.374).

But more moderate light exposures equivalent to sunrise, administered for about 10 hours and centered around the body's temperature trough, nearly stop circadian-rhythm fluctuations in their tracks, Czeisler and his co-workers report in the March 7 NATURE.

The scientists studied 14 men, aged 18 to 29, in a sleep lab. They charted the men's resting biological cycles for about 40 hours, using body temperature and blood levels of the hormone cortisol. In 18 trials, volunteers went through either one 24-hour cycle with eight hours of darkness, seven of indoor-room light and nine of "sunrise" light, or two 24-hour cycles that each included eight hours of darkness, 11 of indoor light and five of sunrise light. Timing of sunrise exposures centered around each man's body temperature minimum -- roughly 5 a.m.

On three trials, regular up-and-down shifts in body temperature and blood cortisol levels nearly stopped. On seven trials, temperture and cortisol cycles shifted unpredictably, with the rhythmic oscillations of one or both partially reduced. And on five trials, only one of the measures displayed a near suppression of oscillations, while the other showed a partial reduction. In three final trials, two men had nearly complete loss of temperature fluctuation and one had partial loss.

Much of the variability in response stems from a difficulty in pinning down the exact point at which light exposure turns off an individual's biological pacemaker, Czeisler says. For instance, one trial shifted a participant's circadian cycle by seven hours, with little effect on rhythmic oscillations. However, just an 18-minute shift in the timing of brightlight exposures during a second trial nearly erased oscillations in that man's body temperature and cortisol.

Biological clocks automatically reset as soon as volunteers left the lab and entered direct sunlight, Czeisler adds. Further work must estalbish whether behavioral changes accompany a near shutdown of circadian rhythms. Two men who experienced circadian suppression seemed to operate "on a more even keel, without any highs or lows" in mood, he remarks.