To define insomnia is difficult, because there can be so many varied causes of sleep disorders. However, an understanding of the Circadian rhythm may shed some light on why a person is having trouble sleeping.
When the world was young, or even only a century ago, humans lived their lives mostly according to the cycles of day and night. We slept just after the sun set and we woke around sunrise, in harmony with the earth’s cycle – the “Circadian Rhythm“. With the advent of all our modern appliances, electricity and artificial light, our whole lives changed. We no longer move and live with the earth’s rhythms, and denied our own biological clock.
Many people nowadays work shift-work. In the United States in 2004, there was an estimated 15,000,000 people who work full-time on irregular schedules, include nightshift, evening shift and rotating shifts or various other irregular schedules. That number has grown much bigger as time moves on.
Being awake and working at these irregular hours is is totally foreign to the human body and the way in which it functions. This can have a devastating effect on our sleeping patterns. Shiftworking alone can be a single cause of Circadian Rhythm sleep disorder. People with this sleep disorder can become unable to get to sleep and to wake at normal times.

Circadian Rhythm and Sleep Disorders
Here is a video from Nightline with Ted Koppel. Program on light, circadian rhythms and circadian sleep disorders including Non-24 hour sleep-wake cycle disorder.
Includes interviews with Dr Thomas Wehr of NIH, Dr Charles Czeisler, Dr Ted Baker, and a person with N24 disorder.
If at all possible, try to live in a way that is more harmonious with the Circadian Rhythm, being out of sync with the natural system can cause many problems – not only insomnia. Being more in line with the Circadian Rythym should make a big difference to helping with insomnia.