Although there are several different degrees of insomnia, about three types of insomnia have been clearly identified: transient, acute, and chronic. Transient insomnia lasts from days to weeks. It can be caused by another disorder, by changes in the sleep environment, by the timing of sleep, severe depression, or by stress. Its consequences - sleepiness and impaired psychomotor performance - are similar to those of sleep deprivation.
Acute insomnia is the inability to consistently sleep well for a period of between three weeks to six months.
Chronic insomnia lasts for years at a time. It can be caused by another disorder, or it can be a primary disorder. Its effects can vary according to its causes. They might include sleepiness, muscular fatigue, hallucinations, and/or mental fatigue; but people with chronic insomnia often show increased alertness. Some people that live with this disorder see things as though they were happening in slow motion, and moving objects may seem to blend together.

Surveys show that people with severe insomnia have a quality of life that is almost as poor as those who have chronic conditions, such as heart failure. In addition to more daytime sleepiness, people with insomnia complain of more attention and memory problems compared to good sleepers. Insomnia can also lead to irritability, mistakes at work, and poorer relationships.
Although stress and depression are major causes of insomnia, insomnia may also increase the activity of the hormones and pathways in the brain that can produce emotional problems. Research indicates that chronic insomnia can increase the risk of developing depression and anxiety. Some investigators are exploring the possibility of preventing psychiatric disorders by early recognition and treatment of insomnia. Even modest alterations in waking and sleeping patterns can have significant effects on a person's mood. In both children and adults, the combination of insomnia and daytime sleepiness can produce more severe depression than either condition alone.

The American Academy of Sleep Medicine (AASM) recommends a number of behavioral methods and prescription medications as the main treatments for insomnia. According to the AASM, these treatment options can improve both quality and quantity of sleep for people with insomnia. Experts agree that behavioral therapies should be the first-line treatment for insomnia. For children in particular, medications should rarely be used as initial treatment. A 2006 study reported that behavioral interventions can provide sustained improvement in over 80% of children with insomnia.