Must a woman cover her hair in bed when she is a niddah? I assume it is standard for a married woman who is a niddah to carry the rules of tzniut into the bedroom; that is, to wear modest sleepwear, etc. But does she also need to keep her hair covered in her bedroom/all night? Does this practice vary by community?
The opinion of Rabbi Moshe Feinstein (YD Vol. II: 75) -- which I've heard quoted by several prominent American Ashkenazi authorities on Laws of Nida -- is that the "modesty" required of a woman in her own home, when she's a nida, is not the full set that would apply for going out in public (e.g. hair covering). Rather, it's whatever she would normally feel comfortable wearing inside when it's just her family around. (I presume when she's not a Nida, her kids don't see her walking around naked -- but very likely they might see her in pajamas.) Hence in virtually all cases, hair covering in the house wouldn't be required. (Yes if you want to be extra pious and holy and stuff ... but as Rabbi Feinstein warned elsewhere, don't be extra holy and pious at the expense of straining your marriage.)
I remember that I read in "Darkei Tahara" of Rav Mordechai Elyahu Zatsa"l that is a h'oumra and even for people who do that, it's not necessary to put again the kissouy roch if it falls during the night.
Sorry but I don't know the Ashkenazi point of view.