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The most reliable workers are those who get seven to eight hours of sleep each night, a new study shows.

Researchers from Finland analyzed the sleep habits and missed work days among 3,760 men and women over about seven years. The workers ranged in age from 30 to 64 at the start of the study.

The researchers found that the use of sick days was associated with the worker’s sleep habits. Not surprisingly, they found that people who did not get enough sleep because of insomnia or other sleep problems were more likely to miss work. But notably, getting a lot of extra sleep was also associated with missed work.

The workers who were most likely to take extra sick days were those who slept five hours or less or 10 hours or more. Short sleepers and long sleepers missed about five to nine more days of work than so-called optimal sleepers, workers who managed seven to eight hours of sleep each night.

The workers who used the fewest number of sick days were women who slept an average of 7 hours 38 minutes a night and men who slept an average of 7:46. The study results were published in the September issue of the medical journal Sleep.