It is not lawful to open the place of shame unnecessarily. There is nothing wrong with keeping it open inside the screen if necessary. Such as during intercourse , during bath or during urination. It has been narrated that the Prophet (peace and blessings of Allaah be upon him) said to one of his Companions , ' Guard your private parts except your wife and slave girl.' The Companions said , ' O Messenger of Allah! If people are in one place ? ' He said , " I will try my best not to let anyone see it." The Companions said , ' O Messenger of Allah! What if someone is alone ? ' He said , " Allah is more deserving than man , that he should be put to shame." -Sunan Abu Dawood , Jame Tirmidhi , Sunan Ibn Majah Here, 'You guard the place of shame with someone other than your wife and slave' - does not mean that the wife and the slave can always be naked. The purpose is to open the place of shame during their meeting or other need , not unnecessarily. Moreover, sleeping naked will cause big problems in case of sudden danger. So caution is recommended. However, I have not received any statement that it would be a sin.