What is behind "I'm so tired"? Fatigue experiences and their relations to the quality and quantity of external stimulation

In the present study, it is hypothesized that both low quality and high quantity of external stimulation are related to elevated levels of fatigue. This is proposed by the Quality-Quantity model for understanding fatigue (QQuF model). The relations between the quality and quantity of external information and the Multidimensional Fatigue Inventory (MFI-20) are examined. Moreover, the role of depression (measured with the CES-D) in relation to the QQuF model is explored. The results show low quality of external stimulation, that is, low "attractiveness of external stimulation," relating to all five dimensions of fatigue. A high quantity of external stimulation, that is, high "experienced overload," related primarily to general and mental fatigue. The QQuF model was only slightly moderated by depression, but depression directly and strongly related to all dimensions of fatigue. It is concluded that fatigue related to low quality of external stimulation can be distinguished from fatigue related to a high quantity of external stimulation. This distinction is useful when considering theoretical issues and treatment of fatigue. (C) 1999 Elsevier Science Inc.