Suppose you work for seven days straight and then take off two days.  If you get tired and recover at the same rate, you'd be at five days tiredness level after the nine days.  This is because after seven days of working, you were at seven-days tiredness level but then you recovered for two days so you're at five-days tiredness level after the nine days.  Now suppose that during the same nine days, someone else worked for five days, then took a two-day break, and then worked for two days.  If they are like you and tire and recover at the same rates, they would also be at five days tiredness level after the nine days.

 Okay, but although you may both be at the same level of tiredness after the nine days, if you were to graph your tiredness level versus theirs over the nine day period, your tiredness level would be greater after day five.  How is this possible if you worked the same number of days during the nine days?  It seems there should be some king of Conservation of Tiredness Law which says this should not be the case.

 Also, as an aside, if we work five days and then take off two days, we never quite make it back to the beginning level of tiredness.  Perhaps working 3.5 days and then taking 3.5 days off could slow down aging.