Energy is a one of the two motives medieval Hero's have. It indicates their need for rest.
Throughout the day, a Hero's Energy will fall at a steady rate until they begin to get Sleepy. Heros
who continue to not rest will eventually become Tired (-30 Focus). Eventually, a Hero who hasn't rested will become Exhausted (-50 Focus) and start to randomly pass out.
Most Hero's rest by napping and sleeping in beds . However, wizards only need to meditate, allowing them to regain their Energy anywhere and at anytime. Some wizards may prefer a bed, as it grants them the "Well Rested" buff. Instead of "sleep" Evil Sims get the "Evil Slumber" interaction, which appears to be no different.