Definition:
A disturbance in a person's sleep patterns, such that one has difficulty falling or staying asleep or experiences sleep that is not satisfying and nonrestorative.
Insomnia can be considered a sleep disorder (according to the DSM-IV) when the insomnia continues for at least one month and causes significant distress and problems in functioning in daily life.
Insomnia (and other sleep problems) is also considered one of the hyperarousal symptoms of PTSD.
You can also learn more about insomnia and other sleep disorders from About.com Guide to Sleep Disorders.
