When daylight-saving sets our external clocks ahead next Sunday on the only 23-hour-day of the year, the body's internal clock will have to make up the difference.

But the body is used to such adjustments; it makes them every day.

Although the modern world marches to the beat of a punctual 24-hour day, the body's internal time clock varies from individual to individual and is usually closer to 25 hours. A person confined to a dark cave or a sleep laboratory without clocks, windows or other time cues will eventually revert to roughly a 25-hour natural time cycle.

"In all of us, our internal physiology is trying to delay about an hour every day," says Dr. Michael Thorpy, director of the sleep-wake disorders center at Montefiore Medical Center in New York. "We're all making a one-hour adjustment anyway."

With spring-forward daylight time, Thorpy says, "essentially that first night we're going to be two hours earlier than our physiology would want."

A host of bodily rhythms follows the circadian ("around a day") cycle keyed to the earth's rotation. The most obvious is the sleep-wake cycle, but circadian rhythms also govern blood pressure, body temperature and the secretion of many hormones, including "virtually every substance you can measure in the blood," Thorpy says.

These circadian rhythms overlap in an intricate pattern of phased relationships, which the brain must continually synchronize.

"As you move sleep, you interrupt all these other relationships," Thorpy says, and the body's internal clock must fine-tune itself.

For most, the shift to daylight time won't be difficult -- the equivalent of one-hour jet lag, as in a flight from Chicago to Washington. Most people are happy to give up an hour on a late-April Sunday in return for added daylight on summer evenings.

"Everyone's going to reset themselves to some degree, come next week," says Dr. Samuel Potolicchio, director of the sleep disorders center at Georgetown University Medical Center. "I don't think anyone will become shellshocked by this. It's only one hour."

Only people who have extremely rigid sleep-wake rhythms will have trouble adjusting, Potolicchio says. He cited the example of a pharmacist treated at the Georgetown center: "Working just two evenings a week threw off his sleep-wake cycle for a whole month."

"You'll be going to sleep an hour earlier the next night and waking up an hour earlier," says Dr. Norman Rosenthal, staff psychiatrist at the National Institute of Mental Health (NIMH). "It's less troubling than typical jet lag."

Body rhythms are governed by two separate internal time clocks, Rosenthal says. One, extremely sensitive to light and darkness, regulates the sleep-wake cycle. The other regulates the secretion of many hormones, including cortisol, the "energizing hormone," which decreases in the evening, drops almost to zero during sleep and then begins to increase before wakeup to help prepare the body to meet the day.

When daylight time shifts the sleep-wake cycle one hour earlier, people keep the first inner clock on schedule merely by going to sleep earlier. But the second internal clock that regulates cortisol may take a little longer to adjust.

"What you have, for about two or three or four days, is a slight change in the relationship between the two clocks," says Rosenthal. "That can lead to some symptoms, such as waking up early or even depression."

Mostly, the symptoms are mild and brief. But in rare cases, even a one-hour time shift can disrupt the body's natural rhythms for a couple of weeks.

Daylight-saving, first suggested by Benjamin Franklin, has been a political hot potato ever since it was introduced in 1918 as a means of saving fuel in World War I. After the war it was immediately repealed, Congress overriding President Woodrow Wilson's veto.

"It was very unpopular," says Carlene Stephens, division of mechanisms curator at the Smithsonian's National Museum of American History.

Year-round daylight time was reinstated during World War II, but it became a local option again from the end of the war until the Uniform Time Act took effect in 1967, establishing a daylight season from late April to late October. A 1972 amendment allows states to exempt themselves from daylight time. For example, the northwest corner of Indiana ignores daylight time. So do Arizona, Hawaii, Puerto Rico, the Virgin Islands and American Samoa.

"I guess they figure they have enough sunshine," Stephens says.

In 1973, during the Arab oil embargo, Congress extended daylight time to make it begin in midwinter for the next two years. For the past decade it has begun on the last Sunday in April and ended on the last Sunday in October.

A Congressional hearing is scheduled today on a bill by Rep. Edward Markey (D-Mass.) to extend daylight time, making it begin in mid-March and end in early November. Such legislation traditionally has been opposed by farmers, who value early-morning daylight more than evening light.

The extension is backed by the Daylight Saving Time Coalition, whose members range from the Amateur Softball Association to the Barbecue Industry Association and the candy industry (which hopes for a boost in Halloween candy sales).

"Not to have daylight-saving in April is absolutely absurd," says Jim Benfield, executive director of the coalition. "We're in daylight-wasting month right now."

April has the earliest sunrises of the year, he notes, which makes it difficult to sleep late on weekends. "April is a bad month for parents with young kids," he says.

Supporters of the extension say daylight time should be scheduled so that its midpoint is the summer solstice (June 21, the longest day of the year) rather than late July, as it is now.

The daylight time extension is also backed by the Retinitis Pigmentosa Foundation, which says 400,000 Americans suffer from degenerative diseases of the eye that cause nightblindness, the inability to see anything but the brightest objects after dark.

"When the sun goes down, their day stops," Benfield says.

A related effect of the shift to daylight time is the increase in exposure to daylight.

"We believe light has a direct effect on mood," says NIMH's Rosenthal, "which may be quite independent of the effect on rhythms."

As with jet lag, the effect of increased daylight varies widely from individual to individual. It elates some people and depresses others.

Rosenthal notes that the peak incidence of hospitalization for depression occurs in spring, and the peak incidence of suicide comes in spring and early summer. The reasons are complex and not fully understood, he says, but they involve both psychological and physical factors.

"It's a time of rapid environmental change," he says, "and some people are better than others at adapting to it."

No one knows for sure how the body developed its natural 25-hour cycle. Some scientists speculate that the margin may have evolved originally to give the body some flexibility to adapt to changes in the seasons.

"Evolution didn't have to worry much about flying to Chicago," says Dr. Richard Wurtman, a neuroendocrinologist at MIT, "but seasonal changes have always been there."

The body's synchronization of circadian rhythms, says Montefiore's Thorpy, is what allows human beings to adapt to a world of changing seasons, changing schedules, changing time. "It's really a fine-tuning system that keeps us functioning."

In fact, jet lag-like symptoms demonstrate not the body's vulnerability but "the fantastic adaptability of human beings," says Frederick Rohles, director of the Institute for Environmental Research at Kansas State University.

"The human organism is a marvelous adapting beast."