There are generally two different labels of insomnia, primary and secondary. Secondary insomnia is insomnia that there is a medical reason for. This type of insomnia should be treated by a physician. This is where one condition is worsened by another disorder. One common cause of secondary insomnia is sleep apnea. This is a disorder that is caused by a difficulty in breathing while sleeping. Symptoms include loud snoring, frequent long pauses between breaths that are followed by a gasping or choking to get air. All of these symptoms are persistently present.
Another common cause of secondary insomnia is Restless Leg Syndrome. This is a condition that occurs mainly at night when a person is trying to sleep when they get some very unpleasant sensations in their legs. The feelings include burning, itching, pulling, or tugging and the only relief is to keep moving the feet or to actually get up and walk around. This can be very irritating as the sensation does not hurt but is annoying and is further aggravated by the inability to sleep.
Circadian Rhythm Disorder accounts for another portion of insomnia and basically is as simple as your sleep cycle not matching up...