Yes Ive had the same problem recently with a pair that were off by 2% love. I couldn't figure out how it happened or why and I had always had enough milk and food so this was not the cause. I cant say I noticed about the sleeping patterns because mainly if my cats are off i just pump them with energy pills.
Something I have noticed though, it is since I upgraded my sim from magnum to standard SL Server...thats when it started. So I wonder if this has anything to do with it.