When it gets harder and harder to keep the eyes open, possible rewards become more attractive whereas possible losses become more and more irrelevant. This has been shown in two experiments with young healthy volunteers in Singapore. Thus, sleepy persons are not only prone to slow reactions, for instance in traffic and other situations where full alertness is demanded. They also take more risks which puts them even more to danger.
The subjects had to perform a gambling task that involved taking risks and expecting rewards. While they were performing the task, their brain has been scanned. In a sleepy state, the brain regions that handle positive expectations were more activated as soon as a risky choice has been made, and the regions responsible for loss prevention turned down. In a second experiment, the choice has been made by a computer and the subjects only watched the outcome. Even then, without active gambling, the positive outcomes evoked more activation of the brain than the negative ones.
Conclusion: Sleepiness has a double threatening impact on decision making, raising positive expectations and lowering the fear of danger.
(Picture by Luna Cruz @ Flickr)