At 3 PM, it's normally 50. If it's 60, I push myself. At 30, I take a nap.
Naps are good way to reenergize and they keep you from dumping more energy on other stuff (e.g. HN, Facebook). Otherwise, I like just watching music videos. Some would say meditation, but paying attention to music is a form of meditation for me.
Sometimes I feel like I need more sleep, and it shows me I'm at 100. So that's a good sign that I already had enough sleep and need to start moving.
Often when I'm very low on energy, I'm too tired to sleep. But there's a burst of energy, the kind you see in tired kids late at night. So the watch would say energy level is at 11 or so and I'd turn off the phone and close my eyes. But when you're tired, often the last thing you want to do is rest.