It sounds like the Well-Rested moodlet glitch. It will get worse over time. There are a few ways around it. It's related to the moodlet, so if you stop your sim from obtaining that moodlet, the invisible negative moodlet that goes with it will go away eventually, too. Wake your sim up before he gets a full night's sleep, just by a little bit. You can always have him nap the rest of the night to get a full night's sleep. A more permanent solution is to choose a different bed. Beds have a + rating when you look at the individual pictures in the catalogue. Any bed with three plusses is safe. I just installed Generations and the glitch disappeared along with it, but if that hasn't happened for you, or you don't have Generations, those other solutions will work.