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Research suggests that people are less likely to come up with convincing lies when they are mentally exhausted. Lying requires your brain to create a story, remember details, and make sure everything stays consistent. When you're tired, your brain has fewer mental resources to manage all of that.
As a result, many people become more direct and truthful without realizing it. This doesn't mean everyone tells the truth when sleepy, but fatigue can make deception more difficult. Your energy level can influence the way you communicate more than you might expect.
© American Psychological Association