As a child I was diagnosed with ADD and put on Ritalin from a young age. As a child I use to have significant insomnia, but did not realize it was due to Ritalin until I finally stopped taking it much later, as I didn't realize my difficulty with sleeping was unusual or qualified as insomnia.
As an adult it seems as if I have an above-average tolerance to stimulants, especially considering I'm slim (if not short) and thus likely in in the lower 50th percentile for mass for males.
In particular I can drink a soda, or coffee, without any difficulty falling asleep or help 'waking up' when tired. I also find that no AD/HD medication appears to have a significant affect on me.
I'm wondering if early exposure to stimulant like Ritalin could have resulted in a higher tolerance to stimulants in general, or if I'm just crazy :).