Hypnagogic Hallucinations
One time my best friend and I were on a voice chat as it was getting pretty late. At one point he said that he should probably head to bed because he was starting to see gory and otherwise disturbing things. Apparently he has hypnagogic hallucinations every so often and just assumed that it was normal to start hallucinating incredibly violent imagery whenever you get tired.
Story credit: Reddit / Palycat