In the name of Allah, Most Compassionate, Most Merciful.

A person may sleep on dry semen; although it is better that a person sleeps where it is clean. However if any substance/particles of the semen can be seen on the clothes or body then they will become unclean and impure. (Durrul Mukhtar, Vol 1, P254)

And Allah knows best.

Sayeedur Rahma

Darul Ifta, Birmingham