I've been thinking about getting some caffeine to add to a bottle of ejuice for those mornings when I can't get my proper caffeine fix. I haven't added caffeine to any of my DIY flavorings but I do know if I vape too much I can get racing heart, shakiness, etc. And I really can't sleep if I vape right before bed unless I take a sleeping pill. Nicotine is a stimulant.