Game logs could be used to measure the effect of rest on performance, if we knew for certain that players rested on days they did not play. The more I think on this, the more I think it's likely that rest will always remain an unknown factor to statistical analysis. One advantage young people have over old people is the ability to sleep. Guys in their early 20s can sleep twelve, fourteen hours a night, no problem. Guys closer to 40 find this harder to accomplish. A father, regardless of age, with a young one will be up many times in the night. Some people do not sleep well in hotel rooms. Others can, but they cannot resist the nightlife. Byung-Hyun Kim, when he came up as a 20-year-old, slept most every non-playing moment of the 2000 season. Other guys use their days off to kill large animals and carry them up stairs. Toward the end of the season, we know a large group of players are suffering from the sleep-affecting side effects of regular stimulant use. So it's likely that some players get more rest in the sixteen hours after a game than others get in the next forty-eight hours.
Someone could generate splits for various players, and that might suggest that they have done better or worse with more or less time off between at-bats, but we could not be sure that this was isolating fatigue as a variable.