In a 22-year study of more than 21 000 adults, Finnish researchers found that participants who habitually slept less than 7 hours or more than 8 hours were more likely to die during the study period.

The findings, published in the journal Sleep, add to evidence linking sleep habits to overall health.

While it's not fully clear why both "long sleepers" and "short sleepers" had a higher death risk, the reasons are likely diverse, said the study's lead author.

"I think there are many subgroups in both short and long sleepers and the reasons probably are different in them," said Dr. Christer Hublin of the Finnish Institute of Occupational Health.

For example, Hublin explained, short sleepers include people who just naturally sleep for a relatively short time, as well as people with insomnia or other problems that rob them of sleep time.

Meanwhile, people who tend to sleep long hours may also have underlying health conditions, or may simply need that much sleep.

"I think that the sleep length, per se, is not the reason for the increased risk of mortality," Hublin said.

The findings are based on a long-term study of 21 268 twins who were asked, among other things, about their usual sleep habits.

On average, Hublin's team found, men who slept less than 7 hours per night were 26 percent more likely to die during the study period than men who caught 7 to 8 hours of sleep. For women, the corresponding figure was 21 percent.

Similarly, men and women who slept for more than 8 hours had death rates that were 24 percent and 17 percent higher.

The researchers accounted for a number of factors that might explain the links. For example, depression can cause people to sleep excessively, but a measure of "life satisfaction" that the study participants completed did not support this as an explanation.

Nor is it likely that undetected major diseases at the study's outset explain the sleep-mortality link, according to Hublin's team. The study period spanned two decades, and the researchers excluded deaths that occurred in the first three years.

While the various reasons for the findings are not yet clear, other studies have linked sleep habits to health conditions as serious as diabetes and heart disease, the researchers point out.