I think that in general, yes, taking care of a baby is tiring. I know I have two young ones and I'm exhausted. I'm sure I haven't slept a full 6 hours in a row in the last 4 years.
But back to you: you say your son sleeps 12 hours straight, but do you? When are you going to bed? How much sleep are you getting. Maybe you need to go to bed early too. :)