Mental performance, like neuromuscular performance, is not perfectly continuous. It is interspersed with short pauses or blocks which have recuperative function. These blocks increase in length and frequency in proportion to the length and difficulty of the task even in the presence of incentive. The fatigue phenomena of one mental task influences another mental task only in proportion to the similarity of the tasks. The same is largely true of muscular tasks when small groups of muscles are concerned. The transfer of fatigue effects from mental to muscular work is slight. As is well known, heavy muscular exercise influences mental performance because of the tendency to fall asleep.