Is masturbation unlawful and sinful if performed during sleep or semi-consciousness, and does it affect one's health in this state?

Masturbation is forbidden. If it occurs during sleep, there is no sin upon the person doing it, for the pen is lifted from the sleeper until he awakens. If it results in the emission of semen, then ghusl (ritual bath) is obligatory; otherwise, it is not. As for its health effects, doctors should be consulted about that.