My p and t are only giving me "when-needed"-medication right now. I get Promethazine.

I took two yesterday because I felt I wouldn't be able to sleep. I am still rather energetic but way less than the last weeks, because I never took to. Anyway it's been more than twelve hours since I took it (and I can feel myself getting more energetic again).

How long does it last? And what else do you think about it?