The relationship between sleep and depressive illness is complex – depression may cause sleep problems and sleep problems may cause or contribute to depressive disorders. For some people, symptoms of depression occur before the onset of sleep problems. For others, sleep problems appear first. Sleep problems and depression may also share risk factors and biological features and the two conditions may respond to some of the same treatment strategies. Sleep problems are also associated with more severe depressive illness.
Insomnia is very common among depressed patients. Evidence suggests that people with insomnia have a ten-fold risk of developing depression compared with those who sleep well. Depressed individuals may suffer from a range of insomnia symptoms, including difficulty falling asleep (sleep onset insomnia), difficulty staying asleep (sleep maintenance insomnia), unrefreshing sleep, and daytime sleepiness. However, research suggests that the risk of developing depression is highest among people with both sleep onset and sleep maintenance insomnia.
Obstructive sleep apnoea (OSA) is also linked with depression. People with depression were found to be five times more likely to suffer from sleep-disordered breathing (OSA is the most common form of sleep disordered breathing). Treating OSA with continuous positive airway pressure (CPAP) may improve depression; a 2007 study of OSA patients who used CPAP for one year showed that improvements in symptoms of depression were significant and lasting.