Most brains are limited to the First Layer, and thus are Shallow Dreamers in essence, distilling their experiences in the Zeroth Layer into their perception of the First, and thus affecting the First within the little bubble of reality it takes up, and vice versa. As it is shallow, however, the change upward is minimal, and not much subjective surrealism gets carried up save for false memories or sensations; common to dreaming.
However, through some mechanism of deepmost, base consciousness, a brain can bottom out through their First Layer into the Second, and thus be considered a Deep Dreamer. Most, if any, only manage to achieve this once before either getting lost in the morass or waking so terrified as to never sleep restfully again—but there are even fewer who take interest in the phenomenon, and cultivate their skill in navigation and pattern-reading. It is said the Deep Dreamers access the Second Layers in REM sleep, during which the connection between the conscious and subconscious is most direct, and can prolong that state to maximize their time out of the conscious world. At usual a session can be three hours before 'surfacing'.
As well, due to their immersion into this more fundamental plane, any perceptions that would normally result in dreams in the First Layer, result in local alterations in the Second, which carry up to the Zeroth in the form of more sustained irregularities of reality. The stronger the perception gotten before sleep, the stronger the irregularity that emerges from the primordial unconscious.
When Deep Dreamers descend, they affect a sort of 'sinkhole' for surrounding minds: sending most into drowsiness, disorientation, microsleep, and inconsistent memory—and allowing other Deep Dreamers to track them on all three known Layers. Such factors allow for competition to be swift and lethal between those who would wish to be solitary in their strange and terrible powers.
Some say that the sensation of 'surfacing' is similar to watching a movie reel burn, whilst being pulled upwards by a hook through the skull.
Every person one meets in dreaming is either real in the Zeroth, or real in the Second.
DEEP DREAMER CRESCENCE
DEEP DREAMER HAXATON WAXWELL
Art credited to Torvenius.