They have a call schedule, and only get called when they're on call. The size of google makes that a more solid rule than it is at smaller companies, I think. Plus they have a more lax work schedule when they're on call, to try and compensate.

I've worked jobs where you really are on 24x7x365 call, and it does suck, and google SRE is not like that.

And hey, it could be worse! Doctors can get called any time, by their most hypochondriac patients...