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More than 70 types of sleep disorders exist. The most common problems are insomnia (difficulty falling or staying asleep), obstructive sleep apnea (disordered breathing that causes multiple awakenings), various movement syndromes (unpleasant sensations that prompt night fidgeting), and narcolepsy (extreme sleepiness or falling asleep suddenly during the day).

Type of sleep disorder, prevalence, and impact vary by psychiatric diagnosis. But the overlap between sleep disorders and various psychiatric problems is so great that researchers have long suspected both types of problems may have common biological roots.

Depression. Studies using different methods and populations estimate that 65% to 90% of adult patients with major depression, and about 90% of children with this disorder, experience some kind of sleep problem. Most patients with depression have insomnia, but about one in five suffer from obstructive sleep apnea.

Insomnia and other sleep problems also increase the risk of developing depression. A longitudinal study of about 1,000 adults ages 21 to 30 enrolled in a Michigan health maintenance organization found that, compared with normal sleepers, those who reported a history of insomnia during an interview in 1989 were four times as likely to develop major depression by the time of a second interview three years later. And two longitudinal studies in young people — one involving 300 pairs of young twins, and another including 1,014 teenagers — found that sleep problems developed before major depression did.

Insomnia and other sleep problems affect outcomes for patients with depression. Studies report that depressed patients who continue to experience insomnia are less likely to respond to treatment than those without sleep problems. Even patients whose mood improves with antidepressant therapy are more at risk for a relapse of depression later on. Depressed patients who experience insomnia or other sleep disturbances are more likely to think about suicide and die by suicide than depressed patients who are able to sleep normally.

Bipolar disorder. Studies in different populations report that 69% to 99% of patients experience insomnia or report less need for sleep during a manic episode of bipolar disorder. In bipolar depression, however, studies report that 23% to 78% of patients sleep excessively (hypersomnia), while others may experience insomnia or restless sleep.

Longitudinal studies suggest that insomnia and other sleep problems worsen before an episode of mania or bipolar depression, and lack of sleep can trigger mania. Sleep problems also adversely affect mood and contribute to relapse.

Anxiety disorders. Sleep problems affect more than 50% of adult patients with generalized anxiety disorder, are common in those with post-traumatic stress disorder (PTSD), and may occur in panic disorder, obsessive-compulsive disorder, and phobias. They are also common in children and adolescents. One sleep laboratory study found that youngsters with an anxiety disorder took longer to fall asleep, and slept less deeply, when compared with a control group of healthy children.

Insomnia may also be a risk factor for developing an anxiety disorder, but not as much as it is for major depression. In the longitudinal study of teenagers mentioned earlier, for example, sleep problems preceded anxiety disorders 27% of the time, while they preceded depression 69% of the time.

But insomnia can worsen the symptoms of anxiety disorders or prevent recovery. Sleep disruptions in PTSD, for example, may contribute to a retention of negative emotional memories and prevent patients from benefiting from fear-extinguishing therapies.

ADHD. Various sleep problems affect 25% to 50% of children with ADHD. Typical problems include difficulty falling asleep, shorter sleep duration, and restless slumber. The symptoms of ADHD and sleeping difficulties overlap so much it may be difficult to tease them apart. Sleep-disordered breathing affects up to 25% of children with ADHD, and restless legs syndrome or periodic limb movement disorder, which also disrupt sleep, combined affect up to 36%. And children with these sleeping disorders may become hyperactive, inattentive, and emotionally unstable — even when they do not meet the diagnostic criteria for ADHD.