I'm a shower-in-the-morning kind of guy. I don't mind it when other people only shower at night, but I have to have a morning shower myself. Even when I take a shower the night before, it's absolutely mandatory that I have another one in the morning. If I don't, I just feel dirty and sticky and weird, even if I'm not. It's also much harder to spike my hair if it hasn't just been shampooed; if I wait more than a couple hours it can be too late.
There's one very nice thing about night showers, though... I get to sleep with clean hair. My hair, in all its spiky, glue-soaked glory, can sometimes become slightly sticky by the end of the night; by the morning, it's a creepy matted mess. That's one reason that I don't spike the hair on the back of my head (the more general reason is just that it's much harder); my pillowcase would have to be changed like every other day.
Those things aside, I just feel a lot more refreshed and awake once I've taken a shower, even if I'm relatively clean from having showered the night before.
It's just a necessity. I need those eighteen minutes each morning.