Related articles

• Brain tells time by morphing
• High-fat diet can mess with body clock
• Babies' internal clock development hindered by artificial lights
• Manic depression may be a rhythm thing
• Einstein is still right on time

gather ye rosebuds while ye may/Old Time is still a-flying." So wrote 17th-century English poet Robert Herrick, capturing the old adage that time flies when you're having fun.

And while our experience of time has intrigued scientists, philosophers and artists for, well, a really long time, vast blanks remain in our understanding of time perception. After all, we can't see, hear or touch time. And time doesn't have a taste or smell.

As physicist Paul Davies of Arizona State University in Tempe points out, we don't really observe the passage of time; what we observe is that later states of the world differ from earlier states that we remember.

Some scientists think our perception of time is a by-product of memory and our senses. Others are searching for specialised timekeeping devices in the brain. One thing is clear: the human mind is prone to time distortions.

After all, time waits for no man, but occasionally it stands still; time heals all wounds but is also the great destroyer; we don't know where the time goes, but there's all the time in the world; time crawls, marches and flies; we make time, save time and kill it.

The quest to uncover our inner chronometer has driven scientists to the extremes of perception: to leaping off towers and living in underground caves. And by focussing on our personal time warps – the ordinary and extraordinary – scientists are unravelling the mystery of what make us tick.

IN JULY 1962, FRENCH GEOLOGIST Michel Siffre lost his grip on time. Deliberately. He descended 130 m underground into a glaciated cave in the southern Alps where he stayed for 60 days. The 23-year-old wanted to know what would happen to his sense of time when cut off from external cues such as clocks, caffeine and cycles of light and dark.

Siffre used a battery-powered lamp to take notes, but the cost of lighting meant he spent most of his time in the dark. His only contact with the outside world was via a telephone he used to let his team know when he lay down to sleep, when he woke, and how much time he thought had passed. Gradually, Siffre lost touch with time.

"When, for instance, I telephone the surface and indicate what time I think it is, thinking that only an hour has elapsed between my waking up and eating breakfast, it may well be that four or five hours have elapsed," he wrote in his diary. At one point, Siffre woke from what he thought was a post-lunch nap. In reality he'd slept for more than eight hours.

On September 14, a rope ladder was lowered into the cave and a confused Siffre was hauled to the surface. His diary entry from hours earlier was dated August 20 – somehow he'd lost 25 days. But while his consciousness experienced a time warp, Siffre's body had been running like clockwork: each day in his cave lasted 24.5 hours, of which he spent 16 hours awake.

Since then, numerous other experiments have confirmed that the human body is governed by a reliable internal clock. From the 1960s, scientists at the Max Planck Institute for Behavioural Physiology in Seewiesen, Germany, studied students isolated in subterranean apartments near Munich.

In 1972, Siffre was observed by NASA scientists during 205 days spent underground in Texas. More recently, scientists have studied volunteers living above ground in 'time-free environments', that is, in apartments without clocks or natural light.

In all of these experiments, after a period of adaptation, the body falls into a precise rhythm of days lasting 24.5 to 26 hours, depending on the individual. This rhythm is set by a biological timepiece scientists call the circadian clock – from the Latin circa ('about') and diem ('a day').

"It's incredibly precise, within minutes per day," says Jay Dunlap, a chronobiologist at Dartmouth Medical School in New Hampshire, USA.

Readers' comments