In reply to your letter of the 8th of Kislev in which you describe the times that you go to sleep and arise, and you ask my opinion if this conduct is correct or whether it should be changed:
The ultimate criteria for the above should be in keeping with the exposition offered by the Alter Rebbe in Hilchos Talmud Torah, regarding a teacher of children. [There the Alter Rebbe states that a teacher] should not stay up so late in the night that [his lack of sleep] will affect the efficacy of his teaching the next day. (See Hilchos Talmud Torah 1:12.)
Understandably, the same applies to one's own study of the Torah [— one should not stay up too late if it will keep him from being alert when he awakens the next day].
Since the amount of sleep a person needs is different from one individual to the other, depending on the nature of his body and the amount of sleep he is used to getting, etc., [it is therefore impossible for me to advise you as to the exact amount of sleep you need].
The statements in the various codes of law regarding the amount [of time] a person should sleep is but a median amount and applies to the majority, for Torah speaks to the needs of the majority, [however, individuals may well vary as to the exact number of hours of sleep they require].
You should test yourself [to see] how many hours of sleep you require, so that when you awaken you will be able to study with the necessary amount of [alertness and] comprehension. Based on this test, you should establish the hours of going to sleep and waking.
It is self-understood that in any case you are to carefully observe the time of the morning Shema, [i.e., that you arise in time to recite the morning Shema], a matter that is of particular import during the summer [when there is a greater possibility of missing the proper time for the recitation of the morning Shema].
(Igros Kodesh, Vol. XIX, p. 89)
... It is obvious that one is to sleep at night (in accordance with both the revealed and esoteric portions of Torah, and as is to be observed as well in the nature of human conduct). Only in unusual circumstances should one nap during the day. (That which is cited in the book
Minhagei HaChasam Sofer requires further examination.)
With regard to [sleeping on Shabbos day,] see the Shulchan Aruch of the Alter Rebbe, end of chapter 281, as well as Pri Etz Chayim, Shaar 16, Chapter 1. This matter requires further investigation.
(Igros Kodesh, Vol. XII, p. 448)
... You write about your disorder, [that in the middle of your learning] you fall into a deep sleep, etc.:
One of the things that may be of assistance is that you study those things in which you have a particular interest; also to change your studies from time to time.
By this I mean that you should not study the same subject matter for many hours on end, or [if you do study the same subject matter for many hours,] do not study it in the same fashion, [e.g., switch from an intensive form of study to a more surface form of study].
Understandably, the above is in addition to your strengthening your health in general, particularly since [being in good health] is important in and of itself, as the Rambam states: "Maintaining a healthy and robust body is an integral part of Divine service."
(Igros Kodesh, Vol. XVIII, p. 7)
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