I always thought that sleepiness, when you stop using nicotine, occurs because it's a stimulant, but there may be more to it than that. Here's a link to an article I found when posting a comment on a blog asking for help regarding feeling sleepy after quitting. I used an NRT and when I stopped it I would get so tired that I would doze off at my workstation and even have to pull over while driving. Once I think I even dozed off for a moment on a busy interstate! It still scares me when I think about that. Here's a link to an article I found that gives another reason. It doesn't mention anything about research but it may still be usefulhttps://www.sayonarasmoking.com/feeling-drowsy-quitting-smoking-heres/
