NEW YORK -- Do naps make you feel even more tired or depressed? Have you ever struggled through a nearly sleepless night only to find yourself surprisingly alert or even euphoric in the morning?

Most people need plenty of sleep to function properly during the day and if they lose too much, it can cause chronic illness, fatigue and anxiety. But for others, there is a paradox. Too much sleep depresses them, and a simple night of sleeplessness can make them feel better at once.

Psychiatrists have recognized for years that depressed patients often sleep long hours and that sleep deprivation can produce dramatic -- but brief -- improvements in their symptoms. But they are not sure why.

By searching for the exact biological relationship between sleep and depression, scientists now hope to learn how depression works and find a better way to treat it.

"We have known for some time that depressed patients often respond very well to a night of sleeplessness," said Ellen Leibenluft, a researcher in the clinical psychobiology branch of the National Institute of Mental Health.

She spoke at a recent seminar on sleep deprivation, conducted here by the American Psychiatric Association. "But it has been hard to find a clinical application that works for more than a day," she said.

Various Strategies Tried

Physicians have used several strategies in attempts to find a useful way to harness sleep deprivation as a therapy. They have combined it with a regimen of drugs; they have experimented with special nap schedules and they have altered fundamental sleep patterns. So far, few of the methods have produced consistent benefits.

Still, it is clear that something important is happening in the brain while people sleep. In various studies of a total of more than 1,500 depressed patients, researchers have consistently found that nearly two-thirds improve enormously after only one night without sleep. The studies rely mostly on the results of extensive mood tests taken by the patients. The benefit has largely been short-lived, however, because most patients relapse as soon as they get some rest. That obviously makes it hard to use sleep deprivation as a long-term treatment.

But the mood-altering effects of sleeplessness can be as powerful as those of any drug.

The opposite has also been reported in many cases: The briefest of naps can send improved patients spiraling back into a world of despair. In one recent study by Joseph C. Wu and William E. Bunney in the psychiatry department at the University of California at Irvine, three-quarters of their depressed patients who were helped by sleeplessness relapsed after a 60-minute nap. One patient suffered a sharp, measurable mood shift after only 90 seconds of deep sleep in the so-called REM (rapid eye movement) phase.

Results like those have led teams of researchers to probe the way sleep affects the brain, hoping an answer may yield important clues or new drugs that could treat one of the world's most common and debilitating afflictions.

"Sleep deprivation has very powerful effects on humans," said Thomas A. Wehr, a leading sleep researcher and chief of the clinical psychobiology branch at the National Institute of Mental Health. "Understanding its effects could help us reproduce them."

Being Awake Improves Mood

Researchers have several theories about why a night without sleep can alleviate depression. Many believe that during sleep a hormone or other chemical substance might be produced or released in the brains of some people and that it intensifies depression.

When they awaken, many depressed people are at their worst, but they generally improve as the day goes on. This suggests that during the day, some chemical associated with depression is broken down by the body and eliminated. That would help explain why simply being awake tends to help patients with their symptoms.

Studies of patients' napping lend support to the theory. Researchers have wondered what can account for the massive mood swings many patients have after only a very short nap. The answer may lie in the idea that sleep causes the release of some chemical that heightens the depression or brings it on.

Although scientists have not pinned down a likely chemical suspect, several hormones such as growth hormone and cortisol are known to be released during sleep and consumed during the waking hours, a pattern that fits the cycle although there is nothing otherwise to implicate them yet.

Another theory suggests that depressed people are somehow unable to regulate their sleep properly. In a world without undue time pressures or alarm clocks, most people simply wake up when their own internal timing mechanism tells them they have had enough sleep.

But depressed people may have the equivalent of a timing mechanism that operates like a broken gas gauge that points to empty when the tank is really full.

Researchers are also beginning to examine the relationship between body temperature, sleep deprivation and depression.

When normal volunteers sleep heated by electric blankets, they produce very little of the thyroid hormone TSH, which many scientists believe helps regulate depression and mood.

This led Wehr and his colleagues to conclude that many of the effects of sleep deprivation are identical to those of lack of body heat.

"When you fall asleep you sweat more and your temperature rises, your TSH levels fall and so do your metabolic heat production levels," said Wehr. "All this is the same as if you got into a hot bath."

Possible Link With Heat

That has made researchers wonder whether they could modify the effects of sleep deprivation by changing the heat to which patients are exposed.

In an experiment still underway at the National Institute of Mental Health, 12 depressed patients were deprived of sleep on at least two occasions. They spent one night in a hot, humid room, and then they spent a night in a cool, dry room.

Most patients did much better in the cool room, bolstering the theory that the heat-like properties of sleep may be partly responsible for depression.

Wehr concluded his presentation with a compelling, though hardly scientific, piece of evidence drawn from author William Styron's recent description of what depression feels like to him.

"Depression takes on the quality of physical pain," Styron wrote earlier this year in Vanity Fair. "Despair comes to resemble the diabolical discomfort of being imprisoned in a fiercely overheated room."

Styron went on to describe how one night as he was awake planning his suicide, the furnace in his house failed. It was bitterly cold, and at about 2 or 3 a.m. as the cold spread across the house, he suddenly felt better.

"Somehow," Wehr said, "in his intuitive way he seems to have latched on to all the elements of these experiments."