LOS ANGELES – It takes people working around the clock to keep modern society functioning. But studies show people who work night or graveyard shifts pay a price.

Working at night is linked to disrupted sleep patterns and an increased risk for several types of health problems, including obesity, heart disease and cancer. The fatigue that results from working odd hours increases the rate of accidents and mood disorders, too.

A new study shows just how sensitive humans are to work shifts. Using a mathematical model, researchers found that the total duration of sleep ranges from 4.5 hours to 8 hours, depending on the start time of a person’s work shift.

The maximum estimated sleep duration occurs among people who start shifts between 9 a.m. and 2 p.m., said the researchers, from Washington State University’s branch in Spokane. The minimum estimated sleep duration occurs when the shift starts between 8 p.m. and midnight.

Minimum on-the-job fatigue occurs when a shift starts at 9 a.m., and maximum fatigue occurs when the shift starts at 11 p.m. Workers who start shifts just after midnight fare better than workers who start at 11 p.m., probably because starting work after midnight allows those individuals to sleep before work.

Shifts that start just before midnight do not allow for a sufficient pre-shift sleep because the timing conflicts with the body’s circadian rhythm. Early evening is a time of day when the body is geared to be alert.

The take-home message of the study is that employers may want to avoid scheduling work shifts that start between 8 p.m. and midnight. The study was presented Tuesday at the Associated Professional Sleep Societies meeting.