Participants were randomly assigned to one of two groups:
The participants were shown multiple images of people with expressions ranging from neutral to very angry.
The participants who had not slept at all the previous night were much worse at recognising the mildly angry expressions than those who had slept well the previous night.
When the sleep deprived participants were allowed to get a full night’s sleep, their ability to recognise angry expressions went back to normal, and was the same as the group who had not missed any sleep.
Sleep deprivation makes us worse at accurately judging facial expressions, especially for anger
You can read the full report for this study here.