An investigation of the relationship between psychomotor activity and serum creatine phosphokinase (CPK) activity showed significant positive correlations regardless of the serum CPK activity. The highest positive correlations were found between preceding evening and nocturnal activity and serum CPK activity, especially in acute schizophrenia, borderline states, and manic-depressive disease. While this finding may reflect the negative correlations between sleep time and serum CPK activity, it is also consistent with the hypothesis that postexercise CPK activity peaks occur approximately eight hours after exercise. Increases in serum CPK activity may also occur subsequent to disordered phasing of psychomotor activity peaks and corticosteroid levels affecting the membrane permeability in skeletal muscle.
Foster FG, Kupfer DJ. Psychomotor Activity and Serum Creatine Phosphokinase Activity. Arch Gen Psychiatry. 1973;29(6):752-758. doi:10.1001/archpsyc.1973.04200060038005