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Not everyone preferred hard pillows: some people complained that ceramic was too cold to sleep on. But sometimes, cutting short sleep was desirable. In the Song Dynasty (960AD–1279AD), scholar and politician Sima Guang 司马光, who is famous for the book ‘Zi Zhi Tong Jian’ 资治通鉴, took advantage of cold ceramic pillows to wake himself up early in the morning so that he would have more time for study.