I'm a big fan of making writing a daily habit. It's a good form of inertia--a truly healthy habit that's difficult to break once formed.
But there are times, once in a while, when you need a rest day from writing.
Running is a helpful analogy. You can run every day for weeks and weeks, but without occasional rest days, most peoples' bodies will break down under the accumulated pounding. A running habit has to be built around occasional rest days so your body can heal and recover.
A writer won't have as frequent a need for a rest day as a runner, but if you put in hours and hours of writing every day, you can certainly wear down your brain just like a runner wears down his body. Your cognitive abilities start to wane and words stop flowing as easily as they should.
Pay attention to how productive your writing sessions are. If you start to see a precipitous decline in your productivity, or if writing suddenly becomes extremely frustrating to you, you're probably due for a cognitive rest day. Give your brain a day off from writing. Put away your pen, your computer, your notebook, and spend the day doing something fun.
Your brain will be back tomorrow, fully rested and refreshed.