Great article about Yoga Nidra or Yogic sleep. Same thing as Dream or Sleep Yoga in Tibetan Dzogchen Buddhism i guess. Remain conscious in Delta brainwaves.
http://www.swamij.com/yoga-nidra.htm
"There has been some degree of confusion circulating about Yoga Nidra and brain waves. Possibly due to the fact that many experience only the Alpha level of brain waves initially, many teach that Alpha is the goal of Yoga Nidra. While the Alpha or Theta levels can be very useful steps along the way, the ultimate goal of Yoga Nidra is the experience of conscious Deep Sleep (Prajna), which correlates with Delta brain waves (True Yoga Nidra practice ultimately leads to awareness beyond any level of physical brain functioning). "
This and Dzogchen are said to be the highest schools and some things are secret. Like how to transform the body into a "light body" at death. Or how to chose your next rebirth
Dzogchen basically teach that you the immortal self is located between thoughts. This is easier to see when getting deeper..
http://en.wikipedia.org/wiki/Dzogchen
"According to Dzogchen teachings, energy of an individual is essentially totally formless and free from any duality. However, karmic traces, contained in the individual's stream of consciousness give rise to two kinds of forms:
forms that the individual experiences as his or her body, voice and mind and
forms that the individual experiences as an external environment.
What appears as a world of apparently external phenomena, is the energy of the individual him or herself. There is nothing external or separate from the individual. Everything that manifests in the individual's field of experience is a continuum. This is the Great Perfection that is discovered in the Dzogchen practice"