Insomnia is not defined by the number of hours of sleep or how long it takes to fall asleep. Individuals vary normally in their need for, and their satisfaction with, sleep. Insomnia may cause problems during the day, such as tiredness, a lack of energy, difficulty concentrating, and irritability.

Insomnia can be classified as transient (short term), intermittent (on and off), and chronic (constant). Insomnia lasting from a single night to a few weeks is referred to as transient. If episodes of transient insomnia occur from time to time, the insomnia is said to be intermittent. Insomnia is considered to be chronic if it occurs on most nights and lasts a month or more.

Women, the elderly, and individuals with a history of depression are all more likely to experience insomnia. If other conditions (such as stress, anxiety, a medical problem, or the use of certain medications) occur along with the above conditions, insomnia is more likely.