Sleep on it". by Maia Szalavitz, Time.com Sep 23, 2013.

"The reasearch, which was published in the journal Nature Neuroscience, showed for the first time that the power of emotional memories - specifically, fearful ones - can be weakened with sleep-based tactics, which offers hope that something as simple as a simple as good night's slumber may reduce phobias and symptoms of post traumatic stress disorder(PTSD).
"Why would sleeping on a fearful experience diminish its power?
Researchers believe that one of the main functions of sleep is to consolidate memories so they can be stored to make room for new memories, therefore freeing up more brain capacity.(Dreams in fact, may be replaying those memories during this processing and storage).
"Other research also suggests that emotional memories may be processed in different ways than neural ones(neocortex), and that sleep tends to reduce emotional intensity. "In general, the idea is that maybe the sleep helps to increase our memory but reduce our worries.", says Bjorn Rasch, noting that this may be why anxiety disorders and depression are often accompanied by sleep problems."
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This seem to make sense. Only our logical thinking brain falls
asleep while the rest is keep working - emotion and senses mostly, without the help of thinking in neocortex, mostly irrational in fear mode. 8 hour sleep a day keeps doctors away.