Since this is a virtual life simulation, and not a VV game, the peeps need to rest. If you're constantly checking on them, they think you want to play with them, and they will be tired from lack of rest.

Besides the suggestions offered above, make sure that you're playing the latest version (1.00.05), which has significant improvements in the peep's energy levels. You can learn about getting the update in Arthur's post, but please pay attention to the warning note, or you could cause your game to revert to a trial version.