Do sugars/carbs make you sleepy or energetic?

I've always heard of the term sugar rush wherein you get a lot of energy when you eat lots of sugar and go all willy nilly, but I've also read that sugars will just decrease metabolism and make you sleepy. The latter is in line with my experiences and It seems as though I just get less energetic when I eat tons of sugar, but what does sugar do really and why do the two contradicting statements exist?