Leeeemeee think, someone correct me if I'm wrong, the seratonin and melatonin levels in our brain is what helps us fall into a natural sleep. Alcohol destroy's that chemical process. After us alkies quit, the brain has to figure out how to restore that chemical process, and that's what takes it so long for us to finally sleep at night ( I may be a little off but I know it's close)... now there are those natural insominacs where it's a completly different and an even more compounded issue.
This is better explained in "Under the Influence".