I'm a sysadmin who pushes back fairly hard when it comes to after hours support. If you don't push back, the company will get used to it, and stop respecting your time. If they need intensive support 24x7, they should have 3 shifts of people working. And no, I'm not the junior admin.
I was the guy that would work whenever, for whatever. I worked hard, and long hours. What'd it get me? Fewer people to help. Less time off. More work. Not significantly more pay. It almost got me divorced, and I got to hear one of my son's first words to me..."bye bye daddy".