Have you ever waken up at night and got caught up in negative thoughts? I once heard that the middle of the sleeping cycle deals with negative emotions. If the negative emotions are not handled or resolved nicely, they will wake up you in the middle of the night to deal with them or appear in a nightmare.
At that moment, it depends on how much mindfulness you have accumulated before. If you can embrace the anger with the mindfulness energy, you can transform negative emotions into wisdom.