The Shulhan Aruch (Orah Haim 4:16) records the tradition mentioned in the Talmud that King David ensured never to sleep for a period of "sixty breaths," so that he would not "experience the taste of death." It appears that once a person sleeps for this duration, his soul ascends to the heavens and his sleep thus constitutes a kind of quasi death. David therefore made a point of sleeping for shorter periods, in order to avoid even the "taste of death."
Of course, it is very uncharacteristic of the Shulhan Aruch, which is a strictly Halachic code, to deal with matters of this nature, concerning the personal practices of Biblical figures like King David. Evidently, the Shulhan Aruch felt that David’s conduct in this regard is instructive and yields practical implications for us. The question thus arises as to how we should apply this Halacha as a practical matter.
The Rama (Rabbi Moshe Isserles, Poland, 1525-1572), in his glosses to the Shulhan Aruch, notes that it appears from the Gemara in Masechet Sukka that David avoided this duration of sleep only during the daytime. At nighttime, however, even David allowed himself to sleep for more than "sixty breaths." Therefore, this Halacha does not apply at all to nighttime sleep. A person may and should sleep at night in accordance with his body’s needs so that he will be able to function properly the next day and serve his Creator with the necessary focus and vigor. It is only during the day when one should preferably avoid sleeping for a period of "sixty breaths."
How long is this period of "sixty breaths"?
This issue is subject to debate among the Halachic authorities, and a very wide range of views exist on the matter. From the writings of Rabbi Menahem Azarya de Fano (Italy, 1548-1620) it appears that he defined "sixty breaths" as the period of three hours and twenty minutes. The Hid"a (Rav Haim Yosef David Azulai, 1724-1806), however, indicates that this period extends for just over a half-hour. Others infer from the fact that the Arizal (Rav Yishak Luria of Safed, 1534-1572) slept for three hours on Shabbat day that the period of "sixty breaths" must extend for at least three hours. Others go so far as to say that the period is six hours long. At the opposite extreme, the Hayeh Adam (Rav Abraham Danzig of Vilna, 1748-1820) held that the period extends for less than one minute.
It should be noted that this discussion does not apply to somebody who sleeps for a particular purpose, such as if he is ill, God forbid, or if he spent the previous night studying Torah until late or plans to spend the following night studying. Clearly, under circumstances such as these, when a person’s condition requires him to sleep during the day, then he may sleep as much as he feel he needs. We speak here of situations where one does not have to sleep during the day, in which case it is preferable to keep one’s daytime sleeping to a minimum, following the example of King David.
Furthermore, this discussion does not apply on Shabbat, when sleeping during the day fulfills the Misva of "Oneg Shabbat" (enjoying Shabbat). We already noted that the Arizal would sleep for three hours every week on Shabbat day, and it has been noted that the Hebrew word "Shabbat" may be read as an acronym for "Shena Be’Shabbat Ta’anug" ("Sleep on Shabbat is enjoyment"). It is preferable to sleep on Shabbat day specifically after the Shabbat meal. On weekdays, however, one should try to avoid sleeping during the day for significant periods of time if he doesn’t have to, as discussed.
Summary: Sleeping on Shabbat day (preferably after the meal) is considered a Misva, but during the week, it is preferable to avoid sleeping for extended periods unless this is necessary.
