Major depression is a common condition that appears to be more frequent in women (lifetime prevalence of 10%-25%) than in men (5%-12%). The prevalence of depression is higher in persons younger than 45 years than in those older than 45 years, though there are many other contributing factors (1).

The course of major depression is highly variable. The average age of onset of major depression is the mid-20s, but the first episode of depression may occur at any age. The number of depressive episodes predicts the likelihood of having another episode (1). Following an initial episode of depression, 50% to 60% of patients will have a second episode. Patients with two episodes of depression have a 70% chance of a third. The chances of subsequent episodes continue to grow with each depressive episode. Remission is relatively unlikely, with only 40% having no evidence of mood disorder at the end of 1 year.

Bipolar I disorder is much less prevalent than major depression, with a lifetime prevalence of only 0.4% to 1.6%, and affects both genders equally (1). Males are more often manic in the first episode of bipolar disorder, whereas females typically have a major depressive episode as the first episode (1). The immediate postpartum period is the most common time for the onset of symptoms in women.

Patients with bipolar disorder tend to have more lifetime episodes than do patients with depressive disorder. In an untreated individual, an average of four episodes occur in a 10-year period (1).

Depression is present in approximately 20% of patients with insomnia, but in only 1% of those without insomnia (2,3). Reports of sleep complaints and fatigue are strongly
predictive of depression (4). The connection between sleep complaints and major depression appears even more pronounced in the young, with depression occurring in 31% of patients with insomnia, 25% of patients with hypersomnia, and 54% of those with both insomnia and hypersomnia as compared to 2.7% of those with no sleep complaints (5). In older adults, insomnia also appears to be predictive of a future episode of depression (6). Resolution of the sleep complaint appears to decrease the chances of a depressive episode (2,7).

In summary, insomnia is predictive of future depression. Sleep disruption is not a risk factor but rather a precursor symptom, which strongly predicts the onset of a depressive episode (8).

Conversely, depression markedly affects sleep as well. During a depressive episode, insomnia is common. The complaints are quite varied, with a decrease in total sleep time (TST), terminal insomnia, sleep-onset insomnia and sleep-maintenance insomnia, disturbing dreams, and nonrestorative sleep. Daytime symptoms of fatigue and excessive daytime sleepiness are also common. The sleep disturbance frequently occurs prior to the onset of the mood episode by several weeks (9).

During an “atypical” depressive episode, patients may report hypersomnia, rather than insomnia. The excessive sleeping may occur during the usual night sleep period or may present as prolonged periods of napping during the daytime, with at least 10 hours of sleep per day. “Atypical” depression is two to three times more common in women than in men. These symptoms are also more common in young patients compared to older adults. Seasonal affective disorder also appears to increase the likelihood of “atypical” features.

As opposed to depression, manic episodes are almost always characterized by a decreased need for sleep and decreased amount of total sleep. The reduction in sleep time has been proposed as a final common pathway in the development of mania (10). There appears to be a bidirectional or cyclic relationship between insomnia and mania, such that insomnia causes mania, which further exacerbates insomnia, and so forth. Sleep deprivation in bipolar patients can precipitate mania (11,12). The insomnia is usually aggressively managed in the manic patient. The mania most likely requires separate treatment. Although the mania is usually associated with insomnia, the depressive phase is usually associated with an “atypical” manifestation of hypersomnia.