Abstract
It has been established that 4.4 to 20% of the general population suffers from a major depressive disorder (MDD), which is frequently associated with a dysregulation of normal sleep-wake mechanisms. Disturbances of circadian rhythms are a cardinal feature of psychiatric dysfunctions, including MDD, which tends to indicate that biological clocks may play a role in their pathophysiology. Thus, episodes of depression and mania or hypomania can arise as a consequence of the disruption of zeitgebers (time cues). In addition, the habit of sleeping at a time that is out of phase with the body's other biological rhythms is a common finding in depressed patients. In this review, we have covered a vast area, emerging from human and animal studies, which supports the link between sleep and depression. In doing so, this paper covers a broad range of distinct mechanisms that may underlie the link between sleep and depression. This review further highlights the mechanisms that may underlie such link (e.g. circadian rhythm alterations, melatonin, and neuroinflammatory dysregulation), as well as evidence for a link between sleep and depression (e.g. objective findings of sleep during depressive episodes, effects of pharmacotherapy, chronotherapy, comorbidity of obstructive sleep apnea and depression), are presented.