Adults who say they sleep between seven and eight hours per night miss fewer work days due to illness than others, according to a Finnish study.

The researchers calculate that if insomnia, apnea and other kinds of sleep disturbances were eliminated, the total cost of worker sick days could be cut by 28 percent.

"Previous studies have already shown associations between insomnia and sickness absence, and there is some evidence regarding the association between sleep duration and sickness absence," said lead author Tea Lallukka, a special researcher at the Finnish Institute of Occupational Health in Helsinki.

But the new study looked at a nationally representative sample of people, which covers many different types of employment, Lallukka said.

She and her co-authors used data from a national survey of Finnish workers over age 30 in 2000, including 1,875 women and 1,885 men.

The participants answered questions about their sleep disturbances, insomnia symptoms, daytime sleepiness and the average hours of sleep they got per 24 hours, and had medical exams to detect mental or physical illnesses.

The researchers also looked up their work absences from 2000 through 2008 in the Social Insurance Institution of Finland database, which registers sickness absences.

Taking age into account, men who reported frequent insomnia symptoms had more than 10 sick days per year, compared with five absences for men who never or rarely had symptoms.

Results were similar for women and for most types of sleep disturbance.

Sleeping between seven and eight hours per night, which included most of the adults in the study, was associated with the fewest sick days per year. Ten percent of women and 13 percent of men reported sleeping an average of six hours per night, and less than 3 percent reported five hours or fewer.

"It is well-known that chronic sleep deficiency causes several daytime impairments," said Borge Sivertsen, a researcher at the Norwegian Institute of Public Health, headquartered in Oslo, Norway.

"Our ability to sustain attention and maintain peak cognitive performance is significantly reduced if we are sleep-deprived over longer periods," Sivertsen said.