There are various types of insomnia, resulting from various causes and needing different approaches. A summary of information an be found in the SDA fact sheet - Insomnia.
Insomnia is the inability to fall asleep, or to stay asleep. The average person needs between 6 to 9 hours of sleep each night, however some people can function well on as little as 5 hours, while others need 10 hours. If you feel that you are having insufficient sleep to function well, you might have insomnia.
Sleep onset insomnia occurs at the start of the night when the person cannot fall asleep. But just as troubling are problems of waking during the night, which is called maintenance insomnia which can have a lot of different causes.
Insomnia may be caused by a medical condition, such as sleep apnea, restless legs syndrome, chronic pain, or it could be associated with environmental factors such as temperature, lights, noise.
Many people experience difficulty sleeping at some time in their life. Transient insomnia is short term and often associated with a specific event where there is anxiety or excitement, or change of time zones. Normal sleep usually returns once the event is over.
Short-term insomnia can last for a few weeks, and is often due to disturbing life events that take longer to resolve. When the insomnia persists for at least three months, it is then called chronic insomnia. Both short-term and chronic insomnia may be helped by relaxation therapy or cognitive behaviour therapy. The best place to start is with your GP, who will investigate medical causes and arrange referrals to the right professionals.
Related to insomnia is Delayed Sleep Phase Syndrome, where the person gets sufficient sleep, but at undesirable times. The person will not be able to fall asleep until the early hours of the morning, but then can sleep all day. This requires reprogramming the body clock, often with light therapy. see SDA fact sheet - Delayed Sleep Phase Syndrome