NEW YORK (Reuters Health) - More than one-third of people aged 16 to 74 suffer from some degree of insomnia, UK researchers report in the journal Sleep.

"Sleep disturbance was a commonly reported symptom in this sample and was associated with significant mental distress," Dr. Robert Stewart of King's College London and colleagues write. "The likelihood is that insomnia remains a substantial but hidden and underaddressed mental health issue."

Several studies have found that insomnia is more frequent among older people, although the reasons for this are not entirely clear. To better understand the prevalence of sleep problems across a wide age range, as well as whether factors associated with such problems might change with age, Stewart's group analyzed data on 8,580 people aged 16 to 74 years.

Overall, the researchers found, 37 percent of the study participants reported some degree of insomnia. Twelve percent had moderate insomnia and 13 percent had insomnia with fatigue. Just 5 percent met medical criteria for insomnia.

Insomnia tended to be most common among the middle-aged people in the study, the researchers found, rather than the oldest. Women and people with less education were more likely to suffer from insomnia, and mental and physical health problems were more common among those with sleep problems.

People who were separated, divorced, or widowed were more likely to report sleep problems, and this association was stronger among younger people.

Sleep problems were more strongly associated with poor physical health-related quality of life among older people, the researchers found. Older people were more likely to report having had insomnia for two years or more, and were more likely to be taking benzodiazepines, a class of sleep-inducing medications that includes Valium and Xanax.

Overall, however, only a small minority of individuals with sleep problems, even those who had suffered from insomnia for the longest time, were taking medication, the researchers found.