This is a glitch that is somehow related to beds with a lower comfort value. They apparently give a flawed "well rested" moodlet. And every time you get this moodlet, your sims mood crashes even further.
As Pam said, it is easily fixed by saving the game, exiting to the main menu and reloading the game. As soon as time starts moving for your sim, his or her mood will return to where it should be.
And a way to avoid it altogether is to simply buy a better bed. Any bed that has three "+" next to the well rested moodlet symbol in the upper right hand corner of the description will not cause this (it won't fix it either, if the bug has already happened). At least some of the beds with two "+" don't cause this either, but while I haven't tried many of them, I can't say for sure if this covers them all...but it most likely does.
Usually, if I'm to the point to where I'm upgrading the beds, I'm going all out.