Which direction should one face upon sleeping?
From North to South: Just as it is forbidden for one to face East or West when having a bowel movement, similarly it is forbidden to sleep facing East or West with one’s wife together in bed, when one has intention to have relations. Thus one must align the bed to face from north to south [or vice versa]. It is proper to be careful in the above even when ones wife is not with him in the same bed. [The bed is to be aligned with its head in the North and its end in the South.]
What to do if one is unable to align the bed from North to South? If one is unable to easily align the bed in a north to south direction, then there is no prohibition for him [to align his bed from East to West and thus] to have relations facing East or West, as in such a case one may rely on the opinion of the Zohar which holds that even initially one is required to face the bed from East to West [and not from North to South].
A married couple is to align the bed in their room from North to South. If this is difficult to accomplish they may align the bed from East to West. [The Mekubalim however rule that one must always align the bed from East to West and so conclude some Poskim.]
Are we particular in this matter even today?
Yes. One is to be particular in this matter even today.
Segula for Male Children
Placing the bed from North to South [is a Segula] for having male children.
Segula for healthy pregnancy
In addition to the above, placing the bed from North to South [is a Segula] for healthy pregnancies without miscarriage.
 Kama 3/9-10. Michaber 3/6; Omitted in Basra.
 Kama 3/9; Michaber 3/6 and 240/17; based on Rashi Brachos 5a
Other Opinions: The M”A 3/7 and Admur 3/10 brings the Zohar who rules the bed is to be aligned from East to West. The Arizal in Shaar Hamitzvos, and Rameh 3 [Harav Menachem Ezaryah], rule like the Zohar. So also concludes Birkeiy Yosef 3/2 stating that even Rashi retracted this ruling in his Sefer Likkutei Pardes. So rules also Shev Yaakov 3 like Zohar; The Olas Tamid 3/6 concludes that one may follow whichever opinion he chooses; Kaf Hachaim 3/16 concludes one is to follow the Zohar and Arizal; The custom of the Munkatcher was like the Zohar. [Darkei Chaim Veshalom 344]; Vayeishev Hayam 2 concludes that most Poskim rule like the Zohar. The M”B 3/11 and Piskeiy Teshuvos 240/31 rule like Admur that one is to initially place it from North to South.
Regarding why we prohibit marital relations from East to West [According to Admur] even though it is surrounded by walls: This question is addressed by the Levush and Olas Tamid 3/6. The M”A 3/7 explains that in a bedroom the Shechina resides, in contrast to a bathroom, and hence one must beware of the above despite there being walls. Alternatively since the only way to have marital relations is within a house therefore the law applies.
 The wording in Admur ibid [which is the wording of Rashi] is “that the head of the bed and its end be aligned one end at the North and the other at the South.” Vetzaruch Iyun at this seemingly superfluous wording.
 Lit. Nachon
 M”B 3/11 in name of Lechem Chamudos
 Parshas Bamidbar brought in Bach; brought in Oar Tzaddikim 27/12
Which end is placed by the East and which by the West? The Arizal [Shaar Hamitzvos Bireishis] writes the bed must be from East to West, with its head by East and its legs [end] by West. However the Rameh writes the head is to be by the west and feet by the East. [brought in M”A 3/7; M”B 3/11]
The reason behind the Kabalistic ruling: The Arizal [Shaar Hamitzvos Bireishis] explains that this alignment corresponds to the directions that Adam was created in, as well as the positioning of the Primordial man of Atzilus, of which the head is by the East which corresponds to Daas, while the feet are by the West. The right hand faces the South which is Chesed and the left hand faces North which is Gevurah. [Brought in Kaf Hachaim 3/16]
 Kama 3/10
 Rav Asher Lemel Cohen, and so is evident from all Poskim ibid that discuss the Halacha. However see Hagahos Kneses Hagedola 3/4 that today the world is no longer accustomed to be careful in this.
 Brachos 5a [stated by Rebbe Chama Bar Rebbe Chanina in the name of Rebbe Yitzchak]; Levush 240/17; Olas Tamid 3/6; This is based on the verse “And in the North their stomach will be filled and they will be satisfied with male offspring.” [Psalms 17/4 ; brought in Brachos ibid]
 Brachos 5a [stated by Rav Nachman the son of Yitzchak]; This is learned from a juxtaposition with the verse: “And her days were complete to give birth and she had twins in her stomach”