The most common sleep complaint in adults is insomnia and is a significant health concern. The criteria for a diagnosis of primary insomnia include difficulty with falling asleep, difficulty remaining asleep, waking up too early, or failing to achieve restorative sleep for a period of 1 month or more while experiencing significant distress and/or impairment in daily functions. One poll found that 64% of participants reported a sleep problem at least a few nights each week, with 41% reporting problems almost every night. Insomnia is 41% more common in women than in men.

Although there are several different degrees of insomnia, three types of insomnia have been clearly identified: transient, acute, and chronic. Transient insomnia lasts for less than a week. 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 less than a month. Chronic insomnia lasts for longer than a month. 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 being unable to sleep, muscular fatigue, hallucinations, and/or mental fatigue; but people with chronic insomnia often show increased alertness.

Patterns of insomnia are:
Onset insomnia - difficulty falling asleep at the beginning of the night, often associated with anxiety disorders.
Middle-of-the-Night Insomnia - Insomnia characterized by difficulty returning to sleep after awakening in the middle of the night or waking too early in the morning
Terminal (or late) insomnia - early morning waking. Often a characteristic of clinical depression.

Poor sleep quality can occur as a result of sleep apnea or major depression. Poor sleep quality is caused by the individual not reaching stage 3 or delta sleep which has restorative properties.

A common misperception is that the amount of sleep required decreases as a person ages. The ability to sleep for long periods, rather than the need for sleep, appears to be lost as people get older. Some elderly insomniacs toss and turn in bed and occasionally fall off the bed at night, diminishing the amount of sleep they receive.

Treatment for insomnia varies depending on the cause. This is why a good workup by a sleep specialist is the best way to make a proper diagnosis and lay out a treatment plan.