Exhausted

I’m in the military with my first kid. He is 4 months old and because of our circumstances me and my wife aren’t living together. Every time I get a chance to go see them I take it and I take care of our baby while giving my wife a break. But I always feel so much more tired at the end of the day then when I work. My job required a lot of physical exertion but I never feel as tired as when I have to take care of my baby. Is taking care of a baby that tiring or is it just me not being used to it?