It doesn’t.
And sometimes, it’s not. I have times when I really should be writing, but instead am, say, on Pottermore (SkyNettle11176, Gryffindor, 14 1/2 inch Holly with Phoenix feather reasonably supple).
My husband knows when I’m slacking, and so does my agent and writing group. They’re also the only people who know when I’m working so hard my eyes are bleeding. The rest of the time…writing is something that people know I do, but they don’t know (or care) when. Only my close friends really understand why I’m such a crap mate. I don’t have time to remember birthdays properly. I hardly have time to remember my shoes. When I’m on a roll, I’m leaving parties just as everyone else arrives. I’m saying goodbye and thanks for dinners when I’m still swallowing my dessert.
For something that doesn’t look like work, and sometimes isn’t, it sure takes up an awful lot of time and thought.
I think, really, that I’m working all the time. But I don’t think, really, that anyone is going to believe me…
My Glamorous Literary Life 
For the creative, work is just as much about processing in the background as it is about hours in front of a keyboard. That’s why we struggle to sleep at times because we fail to fit the creative process into a sensible range of hours! End result – more often than we should we end up looking half dead and appear not to be able to work much when in actual fact we are being our most creative … such is life …