Hamburg: The most difficult time for Ullrich Grubig is in the middle of the night, although that's when there is the least amount of work to do.

While his body wants to sleep, Grubig, a precision mechanic, has to stand at his machine. He must hang on until 5 a.m. when his shift ends. Then he can go home and go to bed. Grubig actually is lucky that the overnight shift ends relatively early.

"When shifts end at 8 a.m., it's usually too bright by the time you get to bed, and that makes it harder to sleep," said Daniel Hicklin, who works at a sleep clinic in Bad Zurzach, Switzerland that advises people who work irregular hours. Daylight signals the body that it's time for activity, not sleep.

At many companies, employees rotate from morning shift to afternoon shift and then to the overnight shift.

"Today, the recommendation is to change every two to three days," said Ingo Fietze of the German society for sleep medicine and sleep research in Berlin. Then the body can adjust better. Ideal is a change from the morning shift to the afternoon shift and then to the overnight shift and then a day off.

"The overnight shift is especially strenuous," said Fietze. Body temperature drops at night and the urge to sleep increases. "You're much more tired and you make mistakes."