Siman 238

The Rambam wrote at the end of chapter 3 of Talmud Torah: "Even though it is a mitzvah to study by day and by night, a person only learns the majority of his wisdom at night. Therefore, one who desires to merit the crown of Torah should be careful with all of his nights and not lose even one of them to sleep, eating, drinking, conversation, and the like, but rather to the study of Torah and words of wisdom. And the sages said: The song of Torah is only at night, as it is said: 'Arise, sing out in the night,' and anyone who occupies himself with Torah at night, a thread of kindness is extended over him by day, as it is said: 'By day the Lord will command His kindness, and in the night His song is with me' (beginning of Lifnei Ideihem), for this song is the Torah. And any house in which words of Torah are not heard at night, fire consumes it (Sanhedrin 99a)," and they derived this in the Gemara from a verse, see there.

Therefore, one must be more careful regarding nighttime study than daytime study, and for one who neglects it, his punishment is great. They said in Eruvin (65a): "The moon was only created for study," and they said there: "The night was only created for sleep," and the Tosafot wrote that this refers to, for example, the nights of the Tammuz solstice; see there. There is one who explained that the language of the Gemara in the first statement, "The moon was only created for study," means that at the beginning of creation the night was total darkness, and since the time the luminaries were hung, it has light by means of the moon; however, for most of the month the moon does not shine the entire night. Therefore, at the time when the night is actual darkness, it is a time to sleep, and when the moon shines, it is a time for study (Maharsha). The great authorities wrote that from Shavuot, on which we read Ruth, wherein it is written: "Lie down until the morning," until Tisha B'Av, of which it is written in Eicha: "Arise, cry out in the night" — "lie down," meaning that the nights are short and there is no need to study at night. And from "arise," which is Tisha B'Av, until "lie down," which is Shavuot — "arise," meaning one must arise and study at night (Magen Avraham and Bach). They wrote that the measure of sleep is eight hours, as it is said: "I would have slept, then I would have been at rest [yinuach li]," and "then" [az] is numerically eight. And some wrote four hours, that when "I would have slept, then [az]" there would be rest for the number of "li," forty hours. The truth is that everything is according to his weakness or strength and according to the number of his days, and everything should be for the sake of Heaven.

Our teacher, the Beit Yosef, wrote: "If one has a fixed law to study such and such per day, and he was occupied during the day and did not complete it—he should complete it immediately at night." Thus far his words. For one who says, 'I shall study this chapter'—he has vowed a great vow to the God of Israel (Nedarim 7a), and one who is accustomed to doing a good thing and his intention is to do so always, it is like a vow, as is written in Yoreh Deah siman 204. Therefore, it is good that for anything one wishes to practice himself, he should say: 'Without a vow' (Magen Avraham). And that which he wrote: 'he should complete it immediately at night,' the intention is not immediately when the night begins, for from where do we have reason to say so, as its source is from Eruvin ibid.: "Rav Aha bar Yaakov would borrow and pay back," but rather the intention is that he should complete it immediately during the night and not leave it for the morrow, and so it is implied in the Tur; see there.