Ever considered just taking a long break (ie 2 hours), spend it wither with some intense physical training & cool down afterwards (ie on some lunch)? In this way, you might be able to keep creativity & concentration high in afternoon too.

It might be just me, but I prefer whole days off compared to shorter more frequent work. Rason - I enjoy trips in nature, and all kinds of full-day or even full-weekend events (hiking, mountains, travel, whatever). The perfect situation for me would be 3 days off, 4 days work. The day I have to work, even for few hours, is already "taken" from me by work.