I can sleep in on Monday morning if I really need to or putter with a paper at 3 a.m. on a weekend. It's hard to quantify hours because your job is to think. Probably like most people, check e-mail. Have to be careful though, e-mail is a great communication tool but it can also suck the hours right out of a day. I typically hit the usual campus suspects of pizza and Thai wraps.
Tension is very much an individual phenomenon in academics.
Tension comes when you're deep in concentration on a new idea and you can't find a good block of hours to finish the thinking required. The intellectual challenge. Discovering new ideas, new ways of thinking and looking at the world. Finding explanations for things that were a mystery, even if you didn't know there was a mystery to begin with. In reality, all of it.