It is forbidden to have sexual relations during the day, as it is forbidden to do so in the light. This is a matter of modesty and privacy, which is a characteristic Jewish trait (Yevamot 79a). The Sages said, “The Jewish people are holy; they do not have sexual relations during the day” (Nidda 17a). Similarly, it is prohibited to have sexual relations at night where there is light (SA 240:11). This does not mean that the room must be pitch dark; rather, if moonlight comes into the room, sexual relations are permitted as long as it does not truly illuminate them. Some people are stringent when possible, closing the blinds or curtains to block out even this light (see MB 240:39).
This prohibition is limited to the conjugal act itself, which must be private and hidden, deep and sublime, as befits this holy mitzva. There is also a concern that perhaps in the light, a man’s wife will seem less attractive to him. The basis of their love is infinite, transcending external beauty, but when they have sexual relations in the light, it becomes limited, dependent on external appearances, and no longer an expression of their endless love. The inner beauty and mystery of their lovemaking is liable to dissipate, which can have a deleterious effect on their relationship. We find a similar phenomenon in the Amida prayer: because it is so sublime and profound, it must be whispered, unlike the other prayers and blessings, which are said aloud. (See Tikkunei Zohar §10, 25a for the comparison of intimacy with the Amida.)
In a darkened room, since it is nightlike, sexual relations are permitted even during the day. If a couple find that their lovemaking is more enjoyable during the day (for example, when they know they will be too tired at night), it is better for them to have sexual relations during the day, in a darkened room. The Talmud relates that this was the custom of King Monobaz’s household, whom the Sages praised for fulfilling the mitzva of ona joyfully (Nidda 17a). Likewise, if a man returns home in the middle of the day from the army or a long trip, he and his wife may darken their room and have sexual relations during the day, even le-khatḥila. Nevertheless, when there is no special need or advantage to having sexual relations during the day, it is best saved for the night, the time that is most suitable and affords the most privacy.
In a time of need, when it is impossible to make the room dark, a Torah scholar may have relations with his wife during the day, provided that they cover their bodies and heads with a blanket. The Sages did not extend this permit to others even in times of need, lest they become lax in this regard (Rema 240:11). However, under pressing circumstances, when a man sees that his urges are overpowering him and he is liable to violate the prohibition of wasting seed (hotza’at zera le-vatala), he may have relations with his wife during the day, on condition that they cover their bodies and heads with a blanket (Ḥokhmat Adam 128:9; SHT 240:25).
At night, if a couple is in a room with lights on or a lit candle, they may not have sexual relations even if they cover themselves with a blanket. Rather, they must put out the light. Even on Friday night, when it is forbidden to put out the lights, they may not have sexual relations even when completely covered by a blanket, as the Sages proscribed relations in a lit-up room (SA 240:11). If the light is coming from outside the room, then the law is the same as during the day; in times of need, a Torah scholar may be lenient and rely on the darkness provided by a blanket, and under pressing circumstances, anyone may rely on this leniency (MB ad loc. 41).
Darkhei Tahara 22:30 states that the couple must cover themselves with a blanket from head to toe, and so states Ben Ish Ḥai, Year 2, Vayera §26. However, Rabbeinu Yehonatan (commenting on Rif, Eruvin 33b) implies that it is enough to cover the head and most of the body.
The conjugal act itself is prohibited in the light, but foreplay is not. We find nothing that prohibits a couple from gazing at one another’s naked bodies. The only exception is that according to some poskim (on Nedarim 20a-b), it is forbidden for the husband to gaze at his wife’s vagina (see section 19 below). On the other hand, it is forbidden to have sexual relations in the light. We can therefore conclude that the prohibition pertains only to conjugal act itself. This is the conclusion of Mishkan Yisrael, part 2, p. 79. Also, see Darkhei Tahara, ch. 22, additional material and responsa §6.
At night, when there is light in the room, it is forbidden to have sexual relations even under the covers (Raavad; Smak; Ritva; SA 240:11 and EH 25:5; Eliya Rabba; MB 240:39). Although there are a few poskim who have ruled leniently in this matter (Ḥida, Petaḥ Einayim on Nidda 17b; Kaf Ha-ḥayim 240:71), here I follow the opinion of the poskim who rule stringently.