St. Maximos the Confessor: When desire grows strong, the intellect in sleep imagines things that give sensual pleasure...

When desire grows strong, the intellect in sleep imagines things that give sensual pleasure; and when the incensive power grows strong, it imagines things that cause fear. 

For the impure demons, finding an ally in our negligence, strengthen and excite the passions. But holy angels, by inducing us to perform works of virtue, make them weaker.

St. Maximos the Confessor