What is the ruling on one who sleeps in his underwear? Thank you.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

It is incumbent upon every Muslim individual to conceal his ‘Awrah (i.e. from the navel to the knees for a man and the entire body for a woman) from those who are not permitted to look at them except one’s spouse. So if sleeping in these clothes does not lead to exposing one’s ‘Awrah, then there is no harm in doing so. However, if this will expose the ‘Awrah to those who are not permitted to see it, then this will not be permissible. Scholars have decided that the means which lead to something prohibited are also prohibited. That is to say if sleeping in these clothes will expose the ‘Awrah to those who are not allowed to look at it, then this will be prohibited. However, if this will expose the ‘Awrah only to your spouse, then there is no harm in sleeping in these clothes. It is better to conceal yourself even if you are alone. The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, was asked: “What of our ‘Awrah should we conceal from others and what should we expose?” He, sallallaahu ‘alayhi wa sallam, said to him: “Conceal your ‘Awrah except from your wife or your female slave.” He said: “O Messenger of Allaah, what if the people are all mingling (in a gathering).” He, sallallaahu ‘alayhi wa sallam, said: “If you can conceal it from everyone then do so.” He said: “What if one of us is alone?” He, sallallaahu ‘alayhi wa sallam, said: “Allaah The Almighty Has more right on us to be shy of Him.”  [Ahmad; Hasan]