Men with chronic insomnia who also sleep less than six hours a night have a higher risk of early death than "normal" male sleepers who get more than six hours of shut-eye, a new study suggests. (Take National Geographic magazine's sleep quiz.)

Insomnia—difficulty falling or staying asleep—is the most common sleep disorder, affecting about 30 percent of people in the U.S., according to the American Academy of Sleep Medicine. Chronic insomniacs are those who have had the disorder consistently for at least a year.

(See "U.S. Racking Up Huge 'Sleep Debt.'")

After decades of mixed findings, the new report shows that insomnia is a "serious disease with significant physical consequences, including mortality," said study