" The evidence shows that human beings function most effectively outdoors at temperatures with a mean average of 60–65 degrees Fahrenheit, with noon temperature 70 average, or a little more. Mental activity is highest when the outside average is 38 degrees, with mild frosts at night. It is important that temperature changes from one day to the next: constant temperatures, and also great swings, are unfavorable—the ideal conditions are moderate changes, especially a cooling of the air at frequent intervals. "

― Paul Johnson , A History of the American People