Common Sleep Disorders

What is insomnia?
Insomnia is too little or poor-quality sleep caused by one or more of the following:

Trouble falling asleep
Waking up a lot during the night with trouble returning to sleep
Waking up too early in the morning
Having un-refreshing sleep (not feeling well rested), even after sleeping 7 to 8 hours at night
Insomnia can cause problems during the day, such as excessive sleepiness, fatigue, trouble thinking clearly or staying focused, or feeling depressed or irritable. It is not defined by the number of hours you sleep every night. Although the amount of sleep a person needs varies, most people need between 7 and 8 hours of sleep a night.

What are the different types of insomnia and what causes them?
Insomnia can be:

Transient (short term) insomnia lasts from a single night to a few weeks.
Intermittent (on and off) insomnia is short term, which happens from time to time.
Chronic (on-going) insomnia occurs at least 3 nights a week over a month or more.
Chronic insomnia is either primary or secondary:

Primary insomnia is not related to any other health problem.
Secondary insomnia can be caused by a medical condition (such as cancer, asthma, or arthritis), drugs, stress or a mental health problem (such as depression), or a poor sleep environment (such as too much light or noise, or a bed partner who snores).

check the link
http://www.4woman.gov/faq/insomnia.cfm#b