Let us suppose that a person sleeps the number of hours per night that is optimal for life expectancy: that is to say, eight hours, both more and less being associated with increased death rates. This means that he has 16 waking hours to fill. Let us suppose that of these, 4 are devoted to tasks that he cannot avoid, such as eating, cleaning, administration etc. He also works on average 6 hours a day. That means that he has 6 hours of disposable time, analogous to disposable income. If, in addition to his basic 15 minutes of exercise daily, he does 30 minutes extra exercise, he spends 8.33 percent of his disposable time on it. This will increase the length of his life by 8 percent. He has therefore gained no disposable time, assuming, that is, that he has performed the exercise solely for its health-giving properties and not for its own sake, that is to say for any enjoyment that he might have had out of it. But people who enjoy exercise will do it anyway, irrespective of its health benefits.
It might be argued that the time he gains will be at the end of his life, when he is retired and therefore has more disposable time on his hands. On the other hand, it is not true that an hour of disposable time at the age of eighty is of the same value as an hour of disposable time at the age of forty. These two considerations probably cancel each other out; and therefore doing exercise is of no value to your life, unless you enjoy doing it.
Be that as it may, the letter in the Lancet ends on a note of irony not frequently encountered in the scientific literature:
We contend, therefore, that the risk of mortality for everyone—prophets included—is 1·0 (1·0—1·0).
We declare that we have no conflicts of interest.
The (1.0 – 1.0) refers to the statistical confidence limits with which the statement is made: in other words the rule is one man, one death. As for the lack of conflicting interest, that speaks for itself.