I think it would be painful to survey how many teachers make a habit of “relaxing” at night with a stack of student writing in their laps and a show they've been wanting to watch on Netflix. I've done this plenty of times myself.
But here is the problem: grading and/or giving feedback on student writing is active, mental work. Trying to grade while watching Netflix is akin to trying to jog while visually reading a book.
If you're jogging and trying to read, then you're constantly switching between the two activities — you can either really slow down your jog so that you can actually see the page and keep it (or your eyes) from bouncing around, or you can really slow down your reading by trying to do it while you're jogging is bouncing everything around. There's this constant lack of quality in either the reading or the jogging, the cost of which would be an hour-long jogging/reading session where you probably did far less reading and jogging than you could have had you just given 30 minutes to exclusively reading and 30 minutes to exclusively jogging.
This is basically what happens in our brains when we do the whole “relaxing while grading” thing. Watching TV is definitely more passive than reading a book, but grading is definitely more mentally active than jogging. So you're sitting here, forcing your brain to try constantly switching between engaging with the show or engaging with the grading task. After an hour of this, you end up, again, with probably much less enjoyment from the show and much less productivity with the grading than you could have gotten by giving an exclusive 30 minutes to each.
I've used watching a show as an example here, but I think the same logic works for other popular recreations like checking one's Facebook timeline or getting on the ol' Twitter. Daniel T. Willingham sums the message up from a research perspective: “The literature is clear on this point. Engaging in any mentally challenging task should be done on its own — not while also watching television or carrying on a conversation.” 
As I've said in more than a couple keynotes or workshops, it's probably not a stretch to say that our grading-while-“relaxing” habits aren't good for us. At best, they decrease the quality of our evenings and increase the amount of time it takes to complete tasks. At worst, I wouldn't be surprised if we're damaging our brains, decreasing our ability over time to deeply engage with mentally challenging work for periods of, say, 60 minutes at a time.
The morals of the story:
- When it's time to relax, relax.
- When it's time to be done working, be done working.
- Embrace the constraints of single-tasking.
- See Willingham's “Ask the Cognitive Scientist: Have Technology and Multitasking Rewired How Students Learn?” here.