Can methadone cause drowsiness

I have been wondering about exactly the same question. There are times when I am overcome by extreme drowsiness that would appear to be in direct correlation to my taking dextroamphetamine. I am on the prescribed drug for ADD and find that at certain times, especially when highly stressed or my mind is racing at lightening speeds, that this reaction occurs. All other times it works exactly as it is supposed to, so am confused as to why this happens. I would very much like to know if there is an explanation for this or if I should be looking for some other reason for my drowsiness?