Once upon a time I could drink a full pot of coffee every morning and feel fine. Now a single caffeinated (sugar-free) soda makes me feel terrible: racing heart, anxiety, shakiness. I've tried exposing myself to more caffeine to get used to it again. No dice. Bodies are weird.
> nasty side-effects like heart racing and insomnia
I've been drinking coffee daily for two decades now and haven't ever experienced heart racing, nor insomnia if I limit my consumption to the A.M.
People do process caffeine wildly differently through. I apparently have genes that make me not very sensitive to caffeine at all, but I know this isn't true for everyone.