Is it permitted for a girl to sleep by herself (alone) in a house?
A person (boy or girl) should not sleep alone in a house, but may do so without fear if a light in the house is left on.
Sources: The Gemara (Shabbos 151b) writes that a person should not sleep alone, and although Rambam and Shulchan Aruch do not mention the prohibition, it is cited by later poskim. According to several Rishonim, the text of the Gemara reads “at night,” and this is generally accepted by halachic authorities (see Magen Avraham 239:7 and Mishnah Berurah 239:9, who do not mention “night,” but see Machatzis Hashekel who explains that the intention is nevertheless sleeping at night). Poskim add that even at night, if a light is on or a candle lit, there is no concern for sleeping alone (Shulchan Aruch Harav, Shemiras Haguf Vehanefesh, Kuntress Acharon 2; Kaf Hachaim 239; Eishel Avraham (Butchatcher) 239), and explain those authorities that prohibit sleeping alone even in the day (as implied in the Zohar, Tazria 45a) as referring to a house without windows, or with curtains drawn, and so on.