Since many keep asking about this topic, below is my original post on a basic level of discernment of the origin of dreams.
Discernment of dreams
According to Saint John Climacus, a simple method exists of discerning the origin of dreams we can have – from where they come: “If we wake up from sleep peaceful, this shows that we have been comforted by the angels unawares. If, on the other hand, we wake up troubled, we are suffering as a result of evil dreams and visions. A dream’s origin is indicated by whether it disturbs us or brings peace” (Met. Hierotheos . The science of spiritual medicine, p. 184).
Dreams from the devil or demons
According to Diadochos of Photiki, dreams originating from the devil or demons “do not keep the same shape, but change from one form to another, alarm the senses, resound with laughter or suddenly become threatening. The figures that appear . . . shout and menace, transform themselves into soldiers and sometimes screech at the soul” (ibid.). These dreams are characterized by color and change. They bring despair, which is always a sign of demonic deception.
Dreams from God
Dreams originating from God “do not . . . [generally] change shape or provoke fear and horror, but bring inexpressible joy and gladness . . . [These dreams are unchanging, with no or minimal color; they lead to] intense prayer, repentance, and a willingness to change” (ibid.).