回答
According to an afterword to the book, her motive for writing the book and the origin of the title came from the episode in which, when Naidaijin (minister of the center) FUJIWARA no Korechika presented to his younger sister FUJIWARA no Empress Teishi and Emperor Ichijo a bundle of paper, which was still expensive in those days, the Empress inquired, 'Emperor used the paper to copy "Shiki (Records of the Grand Historian),' what do you think we could write on this?" to which Sei Shonagon answered, 'it (the book) would be good to use as a pillow.'
巻末の跋文によれば、執筆の動機および命名の由来は、内大臣藤原伊周が妹中宮藤原定子と一条天皇に当時まだ高価だった料紙を献上した時、「帝の方は『史記』を書写なさったが、こちらは如何に」という宮の下問を受けた清少納言が、「枕にこそは侍らめ」と即答した。 - Wikipedia日英京都関連文書対訳コーパス
http://ejje.weblio.jp/sentence/content/When+would+be+good+for+you%3F