The funny thing about mental energy is that it can be managed. I've experiment with this a lot and have found that a somewhat tightly regime of exercising, sleeping, eating (and more importantly watching what I eat), and ever so slightly calibrated caffeine intake, have enabled me to pretty much keep being productive all day.
The biggest effect I've discovered is that you can only do one type of work for about 6 or max 8 hours a day. Then you need to switch to something else for the rest of the day. For instance, if your day is spent programming, then at the very least your side project should be in a different language. Better yet, if it can be something other than programming.