Stanford sleep researcher William Dement said that after 50 years of studying sleep, the only really solid explanation he knows for why we do it is 'because we get sleepy'.

"Answers", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License

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Stanford sleep researcher William Dement said that after 50 years of studying sleep, the only really solid explanation he knows for why we do it is 'because we get sleepy'.