You have described it very well (although your punctuation could be improved), and I understand exactly what you are saying. I don't know of a term for the phenomenon, but I experience it quite often.
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AnonymousWhen you are tired and fighting going to sleep and you dose off sitting down in a chair, the second you fall asleep, you let go of your neck muscle, your head drops forward causing you to wake up.I would just make two corrections. It's "doze off," not "dose off." And "relax your neck muscles" sounds more natural.
khoffIt's "doze off," not "dose off."You're right, of course! However, there may be a case of dosing as well.