The Gemara in Masechet Shabbat (13) establishes the Halacha that forbids a husband and wife from sleeping in the same bed during the time she is a Nidda (forbidden to him due to menstruation). During this time husband and wife must avoid all physical contact, and may not even sleep in the same bed. The two beds must not touch one another, and also the bedding of each bed such as pillows and comforters should not touch the bedding of the other.
The importance of this Halacha emerges from a frightening story that the Gemara (there in Masechet Shabbat) tells of a man who died at a young age. The prophet Eliyahu informed the wife of the sin for which her husband was punished, which, according to one view in the Gemara, was that he would sleep in the same bed as his wife while she was Nidda. Even though the bed was large and they did not engage in any physical contact, the husband was nevertheless severely punished for his violation of this Halacha.
The question arises as to whether this prohibition applies also during the "Ona Ha'semucha Le'vesta" the time when husband and wife must separate given the possibility of the wife becoming a Nidda during that period. For example, if a woman became a Nidda at nighttime twenty-eight nights after her previous onset of Nidda, then she and her husband must abstain twenty-eight nights later, as they must suspect that she may become a Nidda that night. Halacha forbids only full intimacy during this period, but allows other forms of physical contact. The question thus arises as to whether the husband and wife may sleep together in the same bed during the "Ona Ha'semucha Le'vesta," or if this is forbidden just as it is during the time the wife is a Nidda.
Hacham Ovadia Yosef, both in Halichot Olam as well as Taharat Ha'bayit, rules that a husband and wife must, in fact, sleep in separate beds during the "Ona Ha'semucha Le'vesta." Since the concern is that the wife might become a Nidda that night, they must sleep in separate beds to ensure separation in the event that indeed becomes a Nidda. Hence, even though physical contact is permissible during this period, sleeping in the same bed is forbidden. It should be noted, however, that the bedding and even the beds themselves may touch during the "Ona Ha'semucha Le'vesta." Even though once the wife actually becomes a Nidda her bed and bedding must not come in contact with her husband's, during the "Ona Ha'semucha Le'vesta" Halacha requires only sleeping in separate beds, even if they or the bedding touch.
Summary: A husband and wife must sleep in separate beds when the wife is a Nidda and also at the times when intimacy is forbidden due to the likelihood that the wife might become a Nidda. However, whereas during the Nidda period the husband and wife must ensure that their beds and bedding remain fully separate, this is not necessary during the other times when they must sleep in separate beds.
