Even when one is alone is her house, it is proper that she acts modestly, and covers her body. She should not say, “Here I am in the privacy of my own room; who can see me?” for God’s honor fills the whole world. She must cover her body in God’s honor and out of respect for the divine image within her. For men, every body part that one usually covers out of respect when he is among family and close friends should also be covered when he is alone. Concerning women, since the rules of modesty are defined – sleeves until the elbow and skirts until below the knee – it is proper to walk around the house in that manner even when no other person is present.
Similarly, it is inappropriate for girls who live in a same-sex dormitory to walk around in immodest clothing. When getting dressed, it is proper for her to be strict and not change her undergarments in the room. She should change in the bathroom, shower room, or under a cover (Peninei Halakha: Prayer, ch. 7 n. 1).
Regarding a hair covering for a married woman in her house, some say that since the laws concerning hair-covering are not as strict as those governing the covering of one’s body – after all, it is unnecessary for single women to cover their hair yet they are required to cover their bodies – then as there are no strangers in her home, she may walk around without her hair covered. Other poskim are stringent and rule that she must cover her hair even when she is alone in her house (Peninei Halakha: Collected Essays IIICollected Essays III 3:6:18). However, when she is in her bedroom alone or with her husband while in a state of purity, she need not cover her hair.
To explain the idea tzni’ut a bit, it is necessary to begin with the creation of mankind. When created, Adam was pure and clean, both spiritually and physically, and he did not feel any need to cover himself with clothing. However, after his sin, he began to feel ashamed of his nudity. From then on, we all cover our bodies with clothes, especially those parts connected to physical drives and disposal of waste.
The bare body emphasizes in an extreme manner the materialistic and animalistic side of humans. However, the form of the human body, with all the details and intricacies of its limbs, also contains profound and phenomenal allusions to the soul, which the wisdom of the Kabbala discusses in great length. It is the destiny of the body to actualize all those ideas. However, following the sin, our view became more external. At first glance, we only see the corporeal component of the human body, which causes us to forget its spiritual core. Therefore, it is proper to conceal the body, to better emphasize one’s inner spirituality, which is the source of his allure, and allow refined beauty to extend over his whole body. That is what the Sages allude to when they state that modesty in particular preserves beauty, by the fact that it nourishes its eternal root (see Bamidbar Rabba 1:3).
As we have learned, the restraint expressed by modesty emphasizes one’s spiritual component. In addition, modesty greatly contributes to the concentration of the body’s vital energies on the reinforcement of the connection between husband and wife. Modesty turns lust into love. Many people incorrectly think that tzni’ut is meant to dull beauty and the joy of life; however, the exact opposite is true. Tzni’ut preserves one’s beauty and vitality for her spouse, with whom one enters a covenant, in order to increase love, devotion, and life.