When you open your game, what version does it say? The latest version is 1.00.05. If you don't have that version then you need to get it and install it. The tiredness/exhaustion issue was fixed in the later version.
If you do have that version, are you playing mostly in the evening? If so, try switching their day/night on the menu screen. You have to switch it for each family. When you do switch it, you'll find they get up early (like around 5:30 in the evening -- which is 5:30 in the morning for them if you do switch it) and then they are energetic most of the night. Your evening/night I mean.
eta -- here's a post that one of the game developers put on this site that talks about the latest version and how to install it:
http://www.ldwforums.com/ubbthreads/ubbt...o_1_#Post191690