Research suggests opposing alertness effects of mental effort and physical effort: mental effort seems to decrease and physical effort appears to increase subjective alertness. There are indications, however, that physical exercise also leads to feelings of lowered alertness. The well-known multidimensional Thayer alertness scale does not seem to assess physical alertness properly. New items were added to the original scale; these were expected to form a physical factor of subjective alertness. In part 1 of this study, participants filled in the revised Thayer scale before and after a control condition and conditions of physical and mental exercise. Physical exercise only increased feelings of physical fatigue, not of alertness. Mental effort increased feelings of sleepiness. In part 2, a Factor analysis was performed on a larger data set in order to validate the use of a separate physical factor. Indeed, a separate physical factor was found. Besides this physical factor, the analysis revealed the factors “sleepiness”, “calmness”, and “tension”, which have originally been described by Thayer. In conclusion, physical alertness is different from mental alertness. Therefore, an explicit physical factor is required in subjective alertness scales.