Time spent in bed having sex; time spent sleeping in bed; time spent being lazy in bed.
Also written as "sack-time". Comes from old slang term for mattress: sack.
"Damn, so many hot guys around here, but nobody wants to do sack time."
"I just pulled a 12 hour shift. I need to do some serious sack time."
"If you didn't put in so much sack time, you might get something done."