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Not to reveal one’s body:
Not to reveal parts of one’s body that is normally covered 
Due to the above command of modesty one may not reveal those parts of the body which are normally always covered with clothing [if there is no necessity and requirement to do so]. These areas may not be even slightly revealed.
During sleep: Even when one is sleeping in his bed at night, he should cover all of his skin with a blanket.
In a bathroom: Even when one enters the bathroom to do his needs, in which case he must reveal his flesh, nevertheless he is to only reveal that which is needed, and not any more than is needed, even slightly.
What is defined as the “normally covered areas” of the body?
This is dependent on the normal practice of each community and follows the area of the body that is normal to be covered. Thus the face, neck area up until the chest, and from the elbow and down is today considered normally uncovered areas and don’t need to be covered.
Some Poskim rule that the definition of “normally covered” follows the normal dress code in one’s house at times that people may come over and not the way one dresses when he is outside. Thus only those areas of the body that people normally cover when they are at home need to be covered constantly.
May a man wear a short sleeve shirt that does not reach until the elbow?
May a man wear a sleeveless shirt, such as a tank top?
This is forbidden, as it is not common for [orthodox] people to greet people in his house in such dress.
May one wear shorts?
No and so is the custom of all G-d fearing Jews. However some Poskim are lenient in hot areas.
 Basra 2/1
Siddur Seder Netilah: “If one must cover himself even in the bathroom then certainly out of a bathroom one must act with humility and not reveal any of his flesh unnecessarily.”
Kama: The following are the differences between the Kama and Basra regarding these laws of modesty: In the Kama there is A) Much less expounding regarding modesty of revealing parts of the body. B) In the Kama it says only “it is good to be careful” in the laws of modesty while in the Basra it implies that it is an obligation, as will be explained in the Q&A.
 See Q&A for definition.
 Siddur: One may not reveal his flesh if there is no need and requirement.
 For the full details of modesty in the bathroom- See chapter 7 Halacha 4.
 Igros Moshe Yoreh Deah 3/47 and 68; Machatzis Hashekel 4/23 [brought in M”B 4/53; Kaf Hachaim 4/99]
 Igros Moshe ibid; Piskeiy Teshuvos 2/1
 This does not refer to honorable guests but to normal acquaintances and visitors, such as a person that comes to borrow something or request charity. [Based on Igros Moshe 3/68]
 As when one goes outside he may cover more of his body due to the cold and other reasons. However in one’s house, at times that people may come over the only reason for covering the body is due to embarrassment and modesty, and hence only then does it serve as a pure proof as to what areas people consider modest and private. [Igros Moshe ibid]
 See Igros Moshe 3/68; Piskeiy Teshuvos 2/1; Yeshuos Moshe 3/13; Vetzaruch Iyun as to how much of the upper arm may be uncovered. See Beis Baruch 1/32
 See Piskeiy Teshuvos 2/1 footnote 3