RELAX, if you don't take the down time that your body needs then you're just going to pay for it later. I'm a form believer in taking the time each week to relax or do what you want. For me Friday morning I work on personal projects (usually recording a screencast on WordPress) then take the afternoon to go riding. If a client asks me to do things on Friday I simply inform them that I have a standing Friday commitment and can't.
Yes this cuts down on the time I bill in the week, but without the down time my other billable hours are less focussed. So when I bill a client they are getting someone that is relaxed and non-stressed that can put all of their brain power in to the problem at hand.
If you're not taking the time to relax and paying yourself first (with that down time) then start. If you don't know what you would do here is a long list of options to jump start you.