It is forbidden on Shabbat to ask a non-Jew to turn on a light in a dark room. Moreover, if a non-Jew sees a Jew in a dark room and turns the light on for him, the Jew may not benefit from the light. This applies even if the Jew never asked the non-Jew to turn on the light, and even if he had asked him to turn the light on before Shabbat. Additionally, even if the non-Jew is being paid by the Jew, and thus it could be argued that the non-Jew is, in a sense, turning the light on for his own purposes, nevertheless, the Jew may not benefit from the light. The Rama (Rav Moshe Isserles of Cracow, 1525-1572) writes that the Jew does not have to leave the room. Nevertheless, he may not perform activities there that he was unable to perform in the dark, such as reading. Other activities, however, are permissible. (The Aruch Hashulhan gives the example of sitting and drinking, which may be done in the room, since this was possible in the dark.)
This Halacha is very relevant for people who employ non-Jewish housekeepers. If the housekeeper sees that a certain room is dark, and as a favor to the family she turns it on, the family may not benefit from the light.
However, this Halacha applies only if the room was dark. If there was some light in the room, such as from the sunlight coming through the window, and the non-Jew turned the electric light on in order to provide extra light, then one may benefit from this light. Since it was possible to read and perform other activities even without the additional light even if this entailed some level of difficulty it is permissible to benefit from the additional light. This is the ruling of the Shulhan Aruch (Orah Haim 276:4). Hacham Ovadia Yosef adds that if there is some light in a room, one may even ask the non-Jew to turn on the electric light, as long as this is done indirectly, such as by saying, "There is not enough light in the room." As long as one is able to function normally with the original amount of light, he may indirectly ask a non-Jew to add more light.
Later (276:5), the Shulhan Aruch writes that if ones home is very cold, he may ask a non-Jew to turn on the heat. Since people can get sick as a result of exposure to low temperatures, it is permissible to ask a non-Jew to turn on the heat under very cold conditions. If there are young children in the house, then it is permissible to ask a non-Jew to turn on the heat even if the house is not very cold. Given that children are especially sensitive and can become sick if they are exposed to the cold, it is permissible to ask a non-Jew to turn on the heat even under moderately cold conditions. When no children are present, however, this is permissible only if it is very cold, and not if it is just uncomfortably cold.
Summary: If a non-Jew turned on a light in a dark room on Shabbat specifically for a Jew, the Jew may not benefit from that light on Shabbat. He may remain in the room, but he may not perform activities such as reading that he was unable to do without the light. If, however, there was light in the room, such as from a window, the Jew may derive benefit from the additional light, and he may even hint to a non-Jew that he wants the lights turned on. Under very cold conditions, one may ask a non-Jew to turn on the heat on Shabbat.
