Look around. Does it seem like everyone’s having a bad day, blowing you off, or uninterested in you? Then you’re probably under-slept.
UC Berkeley scientists had healthy people react to facial expressions after 24-hours without sleep. And they had trouble telling the difference between a smile, a smirk or a scowl. But after a restful night, the same people had no trouble telling the difference.
Bottom line: If everyone else seems crabbier, YOU need more sleep.