Can Mormons sleep without garments?

While a member of the Church of Jesus Christ of Latter-day Saints (the "Mormon" church) can sleep dressed however they wish, endowed members who have been clothed with the Temple Garment are under covenant to wear the garment whenever possible. The official instruction given by the Church states:

"Church members who have been clothed with the garment in a temple have taken upon themselves a covenant obligation to wear it according to the instructions given in the endowment. The garment provides a constant reminder of the covenants made in a temple. When properly worn, it provides protection against temptation and evil. Wearing the garment is also an outward expression of an inward commitment to follow the Savior.

Endowed members should wear the temple garment both day and night. They should not remove it, either entirely or partially, to work in the yard or for other activities that can reasonably be done with the garment worn properly beneath the clothing. Nor should they remove it to lounge around the home in swimwear or immodest clothing. When they must remove the garment, such as for swimming, they should put it back on as soon as possible."

The instruction later states, "Members who have made covenants in the temple should be guided by the Holy Spirit to answer for themselves personal questions about wearing the garment."

(See Handbook 2 Section 21.1.42 Temple Clothing and Garments which can be accessed here: https://www.lds.org/handbook/handbook-2-administering-the-church/selected-church-policies?lang=eng) So, if a Mormon takes the covenants they have made with God seriously, they should be wearing the garment while sleeping. However, nobody but themselves, God, and perhaps their spouse will know if they don't.