Most of the time I'm not fit for duty....its is due to inadequate rest. This can be caused by something bothering me like a back, shoulder, stomach ache, or someone else in my life that is having problems. I usually just call in sick when the real problem is fatigue/inadequate rest.

Given the new policy on sick letters - more of my calls will be fatigue calls if the end result of "the sick problem" is that I am actually inadequately rested. It can take a week to get over a significant sleep deprivation. Longer, if the underlying cause of the sleep deficit continues past the fatigue call.