Years ago, I had a job where I had a four day week. (Actually four days a week every week, and an extra half day one week in three.) The trouble was, the extra couple of hours per day wore me down; after four days on, I needed an extra day just to get over the cumulative fatigue before I could enjoy the weekend.

The best single thing you can do to reduce working fatigue is to minimize your commuting time. If you commute an hour each way to work, then over a five day week, you've racked up an extra 10 hours -- not to mention the extra expense, and the aggravation periodically inflicted by traffic jams or public transport problems -- and the worst part of it is that it's mostly dead time. (I'll make an exception for commuting to work on foot or by bicycle, as part of an exercise regime.)