What determines the time of onset of fatigue?

There are many potential causes of fatigue. They range from those that cause poor blood supply to body tissues, diseases that affect metabolism, infections and inflammatory diseases, to those that cause sleep disturbances. Fatigue is a common side effect of many medications. While numerous patients with psychological conditions often complain of fatigue (physical and mental), there is also a group of patients whose cause of fatigue is never diagnosed. Lack of energy can also affect the timing of fatigue. What you eat (or don’t eat) can affect how productive you are.