There is no evidence that sleep disorders are a cause of psychiatric disorders. But sleep and psychiatric disorders such as schizophrenia and depression are closely related. Psychiatric disorders are the leading cause of insomnia, the inability to sleep. In addition to insomnia, people with psychiatric disorders have other sleep problems, including sleepiness during the day, and nightmares.
Difficulties with sleep can make psychiatric disorders worse by making the person confused or frustrated, as well as more sensitive to pain and other medical problems.
People who are depressed have a tendency to wake up early, and then cannot fall asleep again. This can make their depression worse, because the amount of sleep a person gets has an effect on their illness. People who don't have a psychiatric illness but suffer from insomnia are more likely to develop a disorder like depression later in their life.
Insomnia can also have a negative impact on treatment of psychiatric disorders. The treatment itself can cause sleep problems; for instance, some medications used to treat these disorders have side effects that can make it difficult for patients to fall or stay asleep.
It's important for people who have a psychiatric disorder to work closely with their doctor for a proper diagnosis and treatment that will allow them to get the sleep they need.