> Their eyes were the clue. The pupil initially constricts when participants are shown the two options. The time it takes for the pupil to subsequently dilate r

> Their eyes were the clue. The pupil initially constricts when participants are shown the two options. The time it takes for the pupil to subsequently dilate reflects the amount of mental exerted. The pupil-dilation times of participants assigned hard tasks fell off significantly as the experiment progressed Maybe tracking pupil dilation time with your webcam can give you a live reading of how tired you are or how much you're exerting yourself at a given task. I'd pay for this.