Question: What is the ruling with regards to having separate clothing for sleeping?

Answer: It is Sunnah to use a separate garment for sleeping.

Sayyidina Abdullah bin Abbas R.A. narrates that he spent a night with his aunt, Ummul Mu'minin, Sayyidah Maymunah R.A. . He noticed that when she prepared Nabi Sallallahu Alaihi Wasallam's bed, she placed a cloth at the head-side. Nabi Sallallahu Alaihi Wasallam returned home after performing Esha Salaah, and used the cloth that was placed there as a lungi (loincloth), removed his day garments, and hung them up. Siratul Khairil Ibaad Vol 7 Page 570

P.S. As a matter of precaution, one should not perform Salaah, wearing the garments that are worn at bedtime. One may be unaware of impurities soiling one's garments while sleeping.

And Allah Knows Best

Yusuf Moosagie