Depression can cause insomnia and insomnia can cause depression. There are many such relationships, like correlations and associations, between disorders. Sleeping disorders and mood disorders happen to be closely linked. It can be confusing, though, determining what the cause is and what the effect is, and if there are any other factors involved.
Insomnia is a condition where a person has problems “trouble falling or staying asleep,” according to the National Heart, Lung and Blood Institute (NHLBI). An abnormal psychology textbook stated that primary insomnia means that this trouble falling or staying asleep, or not feeling rested at all, should not be “related to other medical or psychiatric problems.” The NHLBI adds that secondary insomnia “is the symptom or side effect of another problem.” One of those problems can be depression.
The textbook said that psychological stresses can cause insomnia, and depression can be associated with stress. One article on sleep-deprivation.com states the confusion associated with which disorder causes the other. It adds that overall the root cause must be found and treated. For example, if depression is thought to be the cause of insomnia, then antidepressants should get rid of the insomnia. If insomnia is the root cause, then whatever is causing a sleep disturbance needs to be eliminated.
There seems to be more support of the idea that insomnia actually causes depression, according to research from a few years ago.
The lab director of the study, Michael Perlis, said, according to CBS News: "The assumption has been that if depression is well treated, the insomnia will go away, but this is not the case. It is increasingly clear that you can't ignore chronic insomnia [in patients with depression]. You have to treat it."
It makes sense that not sleeping well can wear people down, drain them and cause stress and depression related to not being able to function and accomplish activities and tasks that are normally completed with ease. It’s difficult for most people to function normally without proper sleep.