This letter was written to Toki Jōnin, a learned and dedicated disciple who lived in Shimōsa Province. In it Nichiren Daishonin stresses the extreme seriousness of the offense of slander and also the importance of embracing the supreme Buddhist teaching. The letter is dated simply the twenty-third day of the eighth month, and though it is generally thought to have been written in the first year of Kenji (1275) at Minobu, no firm conclusion has been reached in this regard. Other opinions are that the Daishonin wrote it in 1276 or even in 1273 while he was still on Sado Island.

In the Daishonin’s teaching, rather than adherence to a specific code of conduct, one’s fundamental posture toward the Mystic Law, or ultimate reality, determines one’s happiness or unhappiness in life. A person who seeks and awakens to the ultimate truth within will attain enlightenment, while one who remains in ignorance of it or even slanders it will continue to be bound by suffering. Hence the Daishonin’s emphasis on exclusive commitment to the Lotus Sutra, which teaches the direct attainment of Buddhahood for all people.

In the last part of this letter, the Daishonin raises a question that had crossed many people’s minds: on the basis of what sort of insight does he dare to criticize such eminent teachers of the past as Kōbō, Jikaku, and Chishō? However, instead of answering this question directly, he simply says, “You had better cut short your sleep by night and curtail your leisure by day, and ponder this!” This passage, from which the letter takes its name, suggests that the most important task of our human existence is to seek out and uphold the correct teaching leading to enlightenment.

https://www.nichirenlibrary.org/en/wnd-1/Content/72