One of the great mysteries of life is why time passes so quickly when you're having fun and so slowly when you're bored.

The secret, according to a Brown University psychologist, may be a series of natural clocks working together in the body.Russell Church says every rhythm of life - from the cycle that tells you it's time for sleep, to the tremble of a shiver - seems to be tied to a natural "clock" in the body that marks time by vibrating like a tuning fork.

Church recently explained his theory of how the body can tick off seconds, minutes and hours with surprising accuracy, and why the internal timer sometimes goes awry.

According to his theory, events may act like drugs to slow down or speed up the natural timekeeper.

Psychologists have known for generations that a biological clock controls the body's sleep-wake cycle. If we were kept in a room, free from any clues about the rising and setting of the sun, our daily cycle would be about 25 hours. Sunlight resets that internal clock to a 24-hour cycle.

The body has other, shorter cycles as well. "Sleep cycles tend to be about an hour and a half," the psychologist said. "In rats, their lick time is consistently about seven times per second."

But how the brain actually measures the passage of time to control those rhythms is not known.

According to one theory, the body's clocks sense the accumulation of chemicals.

Although that theory explains many observations about how biological clocks behave, it has its shortcomings, Church explained. First, the chemicals have not been found. Second, the body would need a way to quickly get rid of the chemical to "reset" the internal clock.

Finally, Church said, the theory doesn't explain why natural rhythms seem to be in multiples of two. For example, people have trouble tapping a different rhythm with each hand unless the second rhythm is twice or four times as fast (or slow) as the first.

Church said there must be something in the body akin to a chorus line of "tuning forks," each vibrating up and down at a different frequency. The body discerns time by the pattern of the forks - which ones are in the "up" position at a particular moment.

The Brown psychologist calls such natural tuning forks "oscillators."

The oscillators have never been found; but the idea, according to Church, explains how our sense of passing time can be altered.