Letters from a Stoic 53 : 7

The opposite holds true of diseases of the soul; the worse one is, the less one perceives it. You need not be surprised, my beloved Lucilius. For he whose sleep is light pursues visions during slumber, and sometimes, though asleep, is conscious that he is asleep; but sound slumber annihilates our very dreams and sinks the spirit down so deep that it has no perception of self.