They had a heart rate monitor at one of the freestyle events which physically affirms what you're saying here. Carlsen's heart rate was barely above resting while his opponents were invariably like they were running a marathon. Even when he was losing, he remained calmer than when his opponents were in normal positions.

I think that should be a normal part of chess competition. It provides some really interesting metadata for spectators. To some degree it also emphasizes the importance of something people don't normally associate with chess - physical conditioning. When your heart is pounding for hours and the cortisol flowing, you literally get physically exhausted.