Does everything ultimately relate to sleep yoga?
1. During sleep yoga, tertons 'travel' to pure lands and receive terma, instructions etc.
2. The progress of all daytime practice, whether mantras or even lhun grub practices, is explicitly assessed by dreams.
3. The practice of the night is a standard feature in Dzogchen practice manuals.
4. Sleep yoga is explicitly emphasized by Dilgo Khyentse, Jigme Lingpa etc.
So is sleep yoga the highest and most direct practice?
Luminosity in sleep is just a natural result of maturation in view. According to Dudjom Lingpa the afflictive consciousness of sentient beings recedes into the ālaya (or substrate) during sleep, but for seasoned practitioners that process starts to dissolve and ignorance in sleep begins to diminish.
There was another dream yoga thread not too long ago where I posted some info from the Dröltig Gongpa Rangdröl which divides the path of no more learning into three stages. In the lesser stage equipoise and post-equipoise are completely mixed, but there is still subtle delusion in sleep. In the medium stage luminosity is stabilized and present both day and night. Regarding the last stage (per Jigme Lingpa): the great yoga of no more learning is also called 'crossing over, without any difference in day or night, to the state of the inseparable three kāyas'
. In Dzogpa Chenpo sem sde this stage is referred to as 'the experience of spontaneous presence transcending the boundaries of ordinary contemplation'.
These three sub-divisions of the path of no more learning span stages [skt. bhūmi, tib. byang chub sems dpa'i sa] 8 to 16. Ergo at the time of the result, wisdom pervades both waking and sleep evenly.
As stated in the concluding remarks (found in song twenty) of Lama Shabkar's Flight of the Garuda
: "When dreaming and being awake are without any difference, that is the time of having actualized the meditation."