Sleeping during daytime:
Sleeping at night versus daytime-Health: Natural sleep is best when it takes place at night. [The Sages state that the nighttime was created for the sake of sleep.] However, sleeping during daytime hours is damaging to the body with exception to those who are accustomed to this. Accordingly, one should not sleep during the day.
Sleeping during daytime-Halacha: It is forbidden for one to sleep during the day for more than Shishim Neshimos [i.e. 60 breaths, 30 minutes or more]. Furthermore, one should not sleep during daytime for even less than this amount of time unless he is unable to learn Torah without this sleep. [However, if one did not sleep well the night before, such as he stayed up at night learning Torah, or if his schedule or nature requires him to sleep during the daytime, then he may do so even for more than 30 minutes. So was the custom of Tzaddikim and Gedolei Yisrael.]
Sleeping during daytime-Kabbalah: Daytime sleep is good for Reshaim but bad for Tzadikim as it causes their Ibbur Neshama of another Tzadik to leave their body. This, however, is with exception to the complete Tzadikim of the generation, and with exception to Shabbos in which it is a Mitzvah to sleep on Shabbos afternoon after the meal, and by doing so one merits the extra Neshama of the Tzadik.
On Shabbos: If one is accustomed to sleep during the afternoons of the week, then he should do so also on Shabbos, as it is considered a pleasure for him. [One may sleep even more than 60 breaths. Furthermore, based on Kabala, it is a Mitzvah to always sleep for some time on Shabbos afternoon, after the Shabbos day meal, and so was the custom of the Arizal.]
 See Encyclopedia Hilchatit Refuit Vol. 2 Erech Sheina p. 674 and 688
 Kitzur SHU”A 32:23; See https://www.health.harvard.edu/staying-healthy/is-your-daily-nap-doing-more-harm-than-good “In addition to reducing sleepiness, naps have been shown to improve memory in the laboratory setting,” says Dr. Suzanne Bertisch, an Associate Physician and Clinical Director of Behavioral Sleep Medicine at Harvard-affiliated Brigham and Women’s Hospital. But the research on napping isn’t all rosy.
“There have been some large epidemiology studies that have suggested both benefits and harms with napping on a population level,” she says. It is difficult to draw conclusions on the individual level. The pros and cons of naps. For example, some studies have found that adults who take long naps during the day may be more likely to have conditions such as diabetes, heart disease, and depression. The urge to sleep during the day may be a sign that they are not getting enough sleep at night, which is associated with a higher risk of developing those chronic conditions. Daytime drowsiness may also be a sign that you are getting low-quality sleep, which may indicate a sleep disorder. In some instances, napping sets up a vicious cycle. You sleep during the day to make up for lost sleep at night, but then you have a harder time falling asleep at night because you slept during the day. “Limiting naps is one strategy to improve overall nighttime sleep,” says Dr. Bertisch. How to nap well. If you do plan to take a nap during the day, here are some guidelines you can follow to help ensure that it won’t interfere with your nighttime slumber. Time it right. The best time to sleep is the early afternoon, when your body experiences a natural circadian dip, says Dr. Bertisch. “If you take a nap in the late afternoon or evening, it will likely be harder to fall asleep later,” she says. Keep it short. Abbreviated sleeps, around 20 minutes, may be best to avoid grogginess when you wake up. Shorter naps can also help to prevent you from having trouble falling asleep that evening. Time it right by setting an alarm.”
 Eiruvin 65a
 Rambam Deios 4:5
 Michaber 231:1; Rama 4:16 regarding Dovid; Rav in Sukkah 26b; M”A 4:16 “Forbidden according to all” [See M”A 4:14 and P”M 4 A”A 15]; Levush 231:1; Rif on Sukkah ibid; Rosh Sukkah 2:14; Mishneh Avos 3:10 and Mefarshim there “Sleeping during the morning is one of the matters that can cause a person to lose his world, as it causes him to miss Zeman Kerias Shema, or as it shows laziness.”
Other opinions: Some Poskim rule that there is no actual prohibition at all to sleep during daytime, and it is a mere Midas Chassidus. [Torah Lishma 383; The prohibition was omitted from Tur, and Rambam -see Aruch Hashulchan 231:3; See opinion of Admur below] Many Chassidic sources praise and encourage sleep during daytime. [See Tzivas Harivash 26-27 p. 65 that one should rest in bed a number of times during the day. Likewise, see Keser Shem Tov 224 to stay up as much as possible at night and sleep a number of hours during the day; Notzer Chesed Kumruna on Mishnayos Avos 3:4 “Be very careful to sleep during day to avoid needing to sleep so much at night”; Darkei Chaim Veshalom 320 “He slept during the day for about an hour”; See also Targum Yonason Shmuel 2 4:5] See Likkutei Sichos 12 p. 254 footnote 50 where the Rebbe questions the above statements of the Besht and explains that perhaps it refers to a case that one desires to use his nights for Torah learning or so he be awake for Tikkun Chatzos. The Rebbe concludes with a Tzaruch Iyun
Tzaruch Iyun from Michaber and Admur: Tzaruch Iyun from Michaber 4:15 regarding if one needs to wash hands after a daytime nap, who does not mention any prohibition against sleeping during the day. Likewise, Admur 4:15 which writes the law of washing regarding one who sleeps 60 breaths during the day, writes of no such prohibition. Furthermore, in Admur 190:2 and Rama 190:1 they rule that one is to sleep on Shabbos afternoon if he is accustomed to sleep during the weekday afternoons. Hence, it is clear that Admur does not hold that sleeping 60 breaths during the day is a standard prohibition as is implied from Michaber above. Vetzaruch Iyun as to whether Admur completely disagrees with the prohibition or simply learns that in many instances it does not apply.
 Machazik Bracha in Kuntrus Achron brought in Shaareiy Teshuvah 4:10; Kitzur SHU”A 2:8; Ketzos Hashulchan 2 footnote 1
The reason: As the 60 breaths refer to the 60 breaths of a horse which takes approximately 30 minutes.
Other opinions: Some Poskim rule that sixty breaths is the equivalent of more than three hours of sleep. [Tiferes Tzevi, brought in Shaareiy Teshuvah ibid, based on that the Arizal slept 3 hours during the day despite being careful not to sleep for 60 breaths] Others rule it is slightly more than three minutes. [Rameh Mepuna, Alfasi Zuta Perek Hayashein] Others rule it is less than a minute. [Chayeh Adam 7:8 writes it is equivalent to the amount of time it takes to walk 100 Amos. This is approximately 1:67 of an hour.] See Shaareiy Teshuvah 4:10; Ketzos Hashulchan 2 footnote 1; Kaf Hachaim 4:55
 See Pela Yoeitz “Sheina”; Pesach Hadevir 231:1; Yifei Laleiv 3 231:1; Binyan Olam 1; Pischeiy Teshuvah 2; Machatzis Hashekel on M”A 4:16; M”B 4:36; Rav Poalim 2 Sod Yesharim 9; Kaf Hachaim 231:1; 239:6; Eiruvin 65; Aruch Hashulchan 231:1 “We saw Gedolei Olam who would sleep for 1-2 hours during daytime in accordance to their necessity”; Shulchan Hatahor 231:1; Yeshuos Chochmah on Kitzur SHU”A 32; Orchos Rabbeinu 1:14; Piskeiy Teshuvos 231:1
 Beis Yosef 239 in name of Maharam; Shut Maryu 6; Rav Poalim 2 Sod Yesharim 9; Aruch Hashulchan 231:1; Darkei Chaim Veshalom 320 slept for about one hour during the day; Tzivas Harivash 26-27 p. 65; Keser Shem Tov 224
 M”A 4:15; Shaar Hakavanos p. 74 and Shaar Hamitzvos Parshas Viaschanon and Peri Eitz Chaim 16:81; Kaf Hachaim 239:5; 4:58
 Admur 290:2; Rama 290:1
 M”A 4:15; M”B 4:36
 M”A 4:15; Shaar Hakavanos 74; Emek Hamelech Hakdama 3:2; Siddur Haarizal; Siddur Kol Yaakov; Kaf Hachaim 231:5; Likkutei Torah Vieschanan “Biur” P. 6; Likkutei Sichos 12 p. 254 footnote 50; However see Kitzur Dinei Shabbos Miluim p. 87 states that nevertheless Chassidei Chabad were particular not to sleep on Shabbos.