I think technically insomnia not only involves having a hard time falling asleep but also includes waking up in the middle of the night and not being able to fall back asleep. The current term for what's been described here is Delayed Sleep Phase Syndrome:

http://en.wikipedia.org/wiki/Delayed...phase_syndrome

I find taking a small dose of melatonin (1-2 mg) about a half-hour before going to bed helps a lot. I don't take it regularly but will do so every so often - especially if I've got to get up extra early in the morning to do a long ride, for example.

It's also really important to wake up at the same time every day (including weekends) to help reset your circadian rhythms. Do this for two weeks and all but the most stubborn sleep problems should resolve themselves.

Scott