New research suggests that as well as being a symptom of mental illnesses such as depression and anxiety, insomnia may also cause them.
Anyone who has gone without sleep adequate sleep for a night, whether as a result of social excesses or factors out of their control such as stress or noisy neighbours, knows that it impacts the way they feel the next day. If we don't get enough sleep we feel tired, irritable, unable to concentrate, and generally not ourselves.
Insomnia is defined by the medical profession as taking longer than 30 minutes to fall asleep on several nights each week which causes problems in daytime functioning. For a clinical diagnosis the problem must have persisted for more at least a month.