Another aspect of chess as a sport is the intense psychological and physiological exertion involved in a competitive chess game, and the crisis after the game. What sports science calls the "stress response process" is at least as powerful in chess as it is in more physical sports. When I say exertion, I am not referring only to the mental gymnastics of moving the pieces in our minds, but also the huge nervous tension that fills you before and during the game, tension that rises and falls with every move and with every idea that passes through your mind while at the board. This tension lasts for hours and a balanced game is a roller coaster of emotions as fortunes change and the battlefield k shifts. Delight can give way to depression in an instant and reverse again a move later, leaving even the most sanguine player exhausted from adrenaline. Managing this nervous energy during each game, and during the ups and downs of an event that may last weeks, is an essential skill for a Grandmaster.

In 2004, winner Rustam Kasimdzhanov walked away from the six-game world championship having lost 17 pounds. In October 2018, Polar, a U.S.-based company that tracks heart rates, monitored chess players during a tournament and found that 21-year-old Russian grandmaster Mikhail Antipov had burned 560 calories in two hours of sitting and playing chess -- or roughly what Roger Federer would burn in an hour of singles tennis.

Robert Sapolsky, who studies stress in primates at Stanford University, says a chess player can burn up to 6,000 calories a day while playing in a tournament, three times what an average person consumes in a day. Based on breathing rates (which triple during competition), blood pressure (which elevates) and muscle contractions before, during and after major tournaments, Sapolsky suggests that grandmasters' stress responses to chess are on par with what elite athletes experience.

"Grandmasters sustain elevated blood pressure for hours in the range found in competitive marathon runners," Sapolsky says.