Mindfulness during sleep I have heard that monatics in many cases sleep as little as 4-5 hours per night. In what way do they train, to reduce the amount of sleep needed for nightly restitution?

Are there methods of training to remain (increasingly) mindful while sleeping?
Once the practitioner attains certain level of mastery in meditation and mindfulness, their sleep acquires a character of meditation. It's not like they don't sleep at all, but they do retain certain level of deep awareness during their sleep.

Specifically, when they see a dream, they know it's a dream. They remain aware of their body lying down on the bed. They remain aware of the content of their consciousness no matter how disjointed it gets during a deep sleep. If something happens around them, they wake up easily to provide their help if needed.

From my personal experience I can say this has gotten increasingly true for me over the years of practice.

As far as specific methods, stopping internal dialogue and mental gossip really helps mindfulness.