That is the general rule and a very important one I think.
Sadly for the first time in my life I didn't manage it. Caught myself nodding of over mid morning the other day but fortunately no dribble on the keyboard.
The culprit - Zyrtec! Now don't let the chemist fool you this stuff can put you to sleep! My chemist promised it was non-drowsy, I of course (stupidly) took it as gospel, and with the ever present streaming eyes, dripping nose and sneezing every few seconds chewed them up like candy looking for a little relief.
Well relief came in the shape of head nodding, struggling to make it to 4.30pm. NOT a good look for a government employee let me tell you! Yeah you can laugh some of us DO actually work :o)