There's a good book out there called "The Willpower Instinct" by Kelly McGonigle. She talks about energy levels a lot. As a busy doc in a cancer hospital energy is a real interest for me; I sometimes sag around 3. Some ideas:
1. Take a 5 min outside walk in mid afternoon.
2. Take 5 min to close eyes, follow breath in/out/in/out (form of meditation)
3. Eat something healthy; sometimes it's low blood sugar.
4. Structure your life so that you use the precious decision making energy for things that matter; make the rest autopilot.
5. Experiment with schedule. I used to slavishly get up and exercise at 6, but that turns out to be a good time for me to make decisions and the afternoon is better for exercise IF I schedule it.
6 Speaking of exercise, I'm experimenting with high intensity interval training (HIT or HIIT). In many studies, doing either a high intensity bike workout (3 intervals of max effort for 20 seconds a few times a week) is just as effective as an hour a day. Google Mike Mosley, The Truth about Exercise http://www.bbc.co.uk/programmes/b01cywtq or visit BodyByScience.net.
I also try to discipline myself to just leave at 5 whether I'm done or not. What I've found is that, first, I'm never done anyway! and second, I get more done with a deadline.