The Index of Physical Capacity (IPC) measured by the Hashimoto step test is one of methods to evaluate a physical fitness. We have conducted an indoor experiment for four years with 40 subjects in total. The purpose of this experiment was to verify the prediction that the work performance of lower score IPC subjects decrease faster than that of higher score IPC subjects. The experiments were continued for 24 hours a day. The subjects had a task and a rest alternately. The results were as follows. The work performance decrease: (1) Gradually after lunch and considerably from midnight to early morning with the lower score IPC subjects (11 people). (2) Considerably from midnight to early morning with the middle score IPC subjects (20 people). (3) Very little from midnight early morning with the high score IPC subjects (9 people).