uprachiasmatic and endogenous are part of everyday conversations for Dr. Lance Kriegsfeld. This is because Kriegsfeld, a neurobiologist at Columbia University, studies circadian rhythms. These words are central to his research.

His work helps explain how the body suffers when people works the graveyard shift, usually associated with the hours between 11 p.m. and 7 a.m. Kriegsfeld explained that humans are a diurnal species, meaning that we are awake during the day and asleep at night.

The word circadian -- Latin for "around a day" -- describes the rhythms that control our bodily functions, such as sleep, digestion and body temperature. The suprachiasmatic nucleus (SCN) located inside the brain, synchronizes these rhythms. In short, the SCN is an endogenous, or internal, biological clock that functions independently from environmental stimulation such as sunlight.

"Even if you were enclosed in a dark room," Kriegsfeld said, "you would still wake up at the same time" as usual.

People who work the graveyard shift, Kriegsfeld said, "are completely going against millions of years of evolution."

"When you try and go against a system that's evolved to a diurnal lifestyle, you are going to run into problems," he said.

ome of these problems include fatigue and a decrease in decision-making skills, which can worsen if changes from day to night are sudden. Kriegsfeld pointed to hospital workers, security guards and air traffic controllers as examples of people who "are usually in a position not only to harm themselves, but harm others as well."

SCN, the human clock     ILLUSTRATION: Lydia Kibiuk

Kriegsfeld mentioned that major accidents at the nuclear power plants Chernobyl, in Ukraine, and Three Mile Island, in Pennsylvania, were indirectly caused by late-night errors committed by fatigued workers.

"Night shift work is associated with decreased productivity," he said.

In addition, night workers usually alter their eating habits. Employees interviewed at the Port Authority Bus Terminal and CBS Radio spoke of having a lunch-sized meal between 2 a.m. and 4 a.m.

When eating food during these hours, Kriegsfeld said, "Your body's not ready for it. You are not going to digest it as efficiently" as you would during the day.

Inadequate digestion, when combined with the fatty fast foods most readily accessible during the early morning, can lead to weight gain and an increased risk for diabetes. This effect intensifies because workers are likely to exercise little, since they spend their free hours sleeping.

s for how people can lessen some of the effects caused by working late-night shifts, Kriegsfeld said it is important to get as much uninterrupted rest as possible.

"Night workers usually receive far less quality sleep" than most people, Kriegsfeld said. He suggested taking melatonin pills, a hormone that is released into the blood stream when it is time for the body to sleep, though he added that no long-term studies exist on the hormone's effects.

Though some workers interviewed said they kept the same sleep schedules on their days off to avoid shifts in their body clock, Kriegsfeld said this was not a permanent solution. "The body still isn't going to be functioning optimally," he said. "You're better off, but you are never going to be perfectly OK."