What is the ruling on sleeping in underwear?

A Muslim must cover their 'awrah (private parts) from anyone for whom it is unlawful to look at them, except for one's wife. If sleeping in certain clothes leads to the uncovering of the 'awrah in front of someone for whom it is unlawful to look, then it is not permissible; because the means to the unlawful are themselves unlawful. However, if the uncovering of the 'awrah only occurs in front of one's wife, then there is no harm. It is preferable to cover oneself even when alone, due to the Prophet's (peace be upon him) saying: "Guard your 'awrah except from your wife or your slave-girl," and "Allah, the Blessed and Exalted, is more deserving that one should feel shyness before Him."