In Book I, stanza 36 specifically refers to sleep. "The sad humour loading the eye liddes" (The Literature of Renaissance England 179) does not mean that Red Cross and the Lady are sad or weeping, but merely that they are tired and go to sleep. Morpheus, mentioned in the third line, is the Greek god of sleep and dreams, and he is described as seeking "out mighty charmes, to trouble sleepy mindes" -- that is, send bad dreams to them.
Stanza 37 continues the discussion of how this god will disturb the sleep of the righteous Red Cross and the Lady. Morpheus chooses from his books of black arts "few words most horrible" and calls to Hecate, the goddess of witchcraft (the wife of the underworld god Pluto). The sleep-god blasphemes heaven, and even calls on Gorgon, the original non-Christian God.
In the thirty-eighth stanza Morpheus calls out small spirits called Sprights, (180) who await his orders. Morpheus chooses two of these sprites to do his bidding. He gives one a message, and keeps the other by his side.
These stanzas set the scene for Red Cross to have the terrible dreams which will be so important in the story. Morpheus, an old pagan god, is shown by Spenser to be, like all pagan worship, an adversary to the good Christian life. At this time in Europe there were still vestiges of old pre-Christian worship, which would have been anathema to the poet of The Faerie Queene.