first at least 4 hrs. is the recommended time minimum when they are tired and you put them to bed....i have found this to be a problem only in the first generation before work areas are upgraded...i combat it by having them nap and or read magazines until their energy level is up.....you just pick them up and drop until it says taking a nap in the activity bar or throw them at the bookcase in the office to get them to read a magazine...depending on how low the energy level is a few times of napping or reading magazine will restore them to about half or a bit over...allowing them to continue work on careers etc.