MR Youngberg et al studied the relationship of caffeine, its metabolite, and insomnia - published in the Journal of Clinical Sleep Medicine. The authors report that nearly 90% of Americans regularly consume caffeine and other studies have shown that caffeine can disrupt sleep but also increase alertness. Having patients reduce or eliminate caffeine consumption, especially within 8 hours of bedtime, is a common recommendation we make to patients with insomnia.
But caffeine does not affect every person the same. This may be a genetic phenomenon, meaning that some metabolize caffeine more rapidly than others. There also could be tolerance to the alerting effects with regular use.
The authors studied the effects of low-to-moderate amounts of caffeine in normal sleepers and those diagnosed with insomnia. The results showed no differences in the two groups with respect to self-reported caffeine intake, caffeine and caffeine metabolite blood levels, and the last time of day caffeine was consumed. The authors concluded that neither caffeine nor its metabolites are significantly associated with sleep disturbance in patients with and without insomnia.
Now, before you insomniacs reach for the coffee pot just before bed, I would argue that perhaps some people can handle caffeine within 8 hours of bed, but others can not. You should experiment with your caffeine intake to see how your sleep quality is affected. Reducing caffeine intake alone is unlikely to "cure" someone's insomnia, but it can be an important part in an overall, tailored behavioral treatment plan.