New York magazine explains the science behind the “fear nap.”

When you look at a psychophysiological level at all of these, one of the reasons that people sometimes get so wiped out and even fall asleep is that whenever you’re put up to a lot of mental tasks or physical tasks, it uses up glucose in the brain,” says Reisinger. “Glucose is essentially sugar, and it gives you the energy you need. My thinking on [the fear nap] is that the most likely culprit, if you will, is depleted amounts of glucose in the brain.”

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