Notice a colonist losing trait points while asleep. It was the construction trait (highest the colonist had)
Skills naturally decay if you are not actively using them. As soon as you stop doing construction, even in a minute or two, the skill points start going down slowly.
As NolanSyKinsley says, there is a skill decay feature, whereby skills will decrease when not being used (although not below a certain level IIRC. Although that might have changed). Was it this you were observing? or something else?