Al-jawab billahi at-taufeeq (the answer with Allah's guidance)
Sleep is a kind of death. The soul leaves the body and goes to where Allah Ta'ala wills, and discovers unseen matters in the world of souls, which could not be discovered during consciousness.
Allah Ta'ala Says (interpretation of meaning): {It is Allah Who takes away the souls at the time of their death, and those that die not during their sleep. He keeps those (souls) for which He has ordained death and sends the rest for a term appointed. Verily, in this are signs for a people who think deeply.}[39:42].
A person who is asleep could see in his sleep strange things. If he sees what pleases him, he should not disclose it and he should not inform anybody else except whom he loves. And if he sees a nightmare, then he should blow to his left side and seek refuge in Allah from the evil of Satan. In this way it will not harm him and he should not inform anybody else about it. This ruling has been reported in sound Ahadith of the Prophet (Sallallahu Alaihi wa Sallam).
