I used to run several sessions a week and play in a couple of bands, and fatigue -or more accurately, boredom- set in eventually.  I backed off and now only play when I feel like it; there are still tunes that I'm thoroughly sick of, but I no longer feel obliged to play them.  When I enjoyed it, it never made me tired; I suspect that variety (and taking time off when you need it, as Rich suggested) may be more important than vitamins!