The results of an interesting sleep study were just published in Time Magazine.


They found that people who were allowed to take a nap during the day, in which they reached REM sleep, found it easier to identify positive emotions in people's expressions, while people who didn't have REM sleep or didn't get to sleep at all were more likely to ID negative expressions.


The researchers suggest an evolutionary explanation:


If you're tired, you need to pay more attention to things that are immediately dangerous to you. If you're well rested, you're likely to look for things that will help you in the long-term, like finding good, nutritious food or a mate, which require knowing which things make people happpy.



Related info:


The Evolutionary Theory of Dreams
Positive Emotions in Dreams
Negative Emotions in Dreams