Except for an occasional yawn and droopy eyes, most students' bodies can handle pulling an all-nighter once in a while.

Students have enough flexibility to overcome an occasional all-night study session, said Frederick Brown, an associate professor of psychology. Younger adults adjust more easily than older people, he said, though he added that successive all-nighters can challenge the body's systems.

"Usually in a one-time situation, you become more alert," said Rocco Manfredi, director of the University's Hershey Medical Center's sleep disorders clinic.

When the body is deprived of sleep, its circadian rhythm -- or biological clock -- is disrupted, Manfredi said. Once this occurs, people need about three or four nights to re-establish their regular sleep rhythms, he said.

"Maintaining a consistent pattern is most important," Manfredi said.

Art Titzel (junior-education) said he only pulls all-nighters when he has a paper due the next day. Although he feels tired the next day, Titzel said catching up on sleep takes him about a day.

"I think you do better on an exam if you don't stay up all night," said Suzanne Vincent (senior-secondary education), adding that she refuses to pull all-nighters.

To ensure healthy sleeping habits, Manfredi said, a person should average 6½ to 7½ hours of sleep each night. However, too much sleep may result in lethargy, he said.

The longer a person goes without sleep, the more the body wants sleep, Brown said. Sleep deprivation increases chances of illness and death rates, Manfredi added.

In addition, sleep deprivation reduces hand-to-eye coordination and decreases perception and reaction times, Manfredi said. The Three Mile Island and Chernobyl disasters occurred because workers didn't have enough sleep, he said.

Brown agreed, "One is not as sharp during the nighttime hours as during the daytime."

What time a person sleeps also affects how a person feels the next day, Manfredi said. Losing sleep during the latter hours of the night may cause a person to feel more energetic when they wake up, he said.

The body experiences rapid eye movement sleep during the last few hours of sleep. REM sleep is not as deep as sleep earlier in the night, he said, so getting up earlier than usual may provide a temporary energy increase.