I am 42, male. For the past few months, I sometimes get sudden tiredness, drowsiness and a heavy feeling for a few hours. The strange thing is that I may be completely normal and active for several days, then suddenly it happens again. During the episode: BP is normal. Pulse is normal. Blood sugar is normal. I am not physically weak and can still walk/exercise normally. No unexplained weight loss. ECG, echo, TMT, brain scan and recent blood tests have been normal. Has anyone experienced this kind of intermittent tiredness/drowsiness? What was the cause in your case?