Dreamscape

The dreamscape is the interior of the mind. Every living, animated thing has a dreamscape. Dreamscapes are split into five zones or "rings" of sanity/consciousness: sunlight, twilight, midnight, lower midnight, and hadal. Clairvoyants can consciously access their own dreamscapes while amaurotics may catch glimpses when they sleep.

Description
Jaxon Hall describes the dreamscape to Paige Mahoney this way: "Everyone has a dreamscape, you see. An illusion of safety, a kind of locus amoenus. You understand. Voyants have colored dreamscapes. The rest have black-and-white ones. They see their dreamscapes when they dream. Amaurotics, consequently, dream in monochrome."

The locus amoenus is a peaceful and idyllic place, usually a garden, lawn, or woodland, where safety or comfort can be found. Dreamscapes can take virtually any form, from fields and lakes to rooms, libraries and alleys, but only clairvoyants have color in their dreamscapes.

Your spirit lives, ideally, in the very center of your dreamscape. Clairvoyants, including Rephaim, can consciously access their own dreamscapes when they want to feel safe, and see it as if it's a real place. However, the vast majority of them cannot walk away from the central zone without causing massive damage to their sanity. Here's where Paige's gift comes in. Eliza asks Paige to draw her a picture of the dreamscape from a bird's eye view. She would have produced something like the diagram below.

Zone 1
The first ring of consciousness is the sunlit zone. In order to stay sane, most people need to keep their spirit fixed in this zone. The silver cord, the link between body and spirit which keeps a person anchored within their physical form, prevents them from moving away from this zone. It forms a kind of wall around that zone. The spirit staying the sunlight zone is equated with a healthy mind. If a spirit were to drift beyond this zone on a permanent basis, the individual would begin to lose their sanity.

Zone 2
The second ring of consciousness is the twilight zone. When someone is particularly stressed, the silver cord becomes more flexible, and the spirit is able to drift into the twilight zone. An amaurotic would associate this experience with having a nightmare.

Zone 3
The third ring of consciousness is the midnight zone.

Zone 4
The fourth ring of consciousness is the lower midnight zone.

Zone 5
The fifth and final ring of consciousness is the hadal zone. Once you reach the hadal zone, you're too far gone to function. This happens to the Underguard in Chapter 1 of The Bone Season. Paige somewhat accidentally uses her gift to shove his spirit into his hadal zone, effectively destroying his sanity. A specter is a manifestation of a person's fears or anxieties and they dwell in the halal zone.

Dream-Form
Your projection of yourself in the dreamscape is known as a dream-form. It is the shape taken by a spirit when it's within the confines of a dreamscape. In the æther, a spirit has no form. It's simply a "faceless glimmer," as Paige calls it. You can't see your own dream-form, but you can imagine it looking a certain way if the situation calls for it.

Trivia

 * Samantha Shannon first got the idea for dreamscapes when she was writing in her mid-teens. It was a type of stage on which recurring dreams played out. She developed it fully when she was studying medieval poetry at university and discovered the term locus amoenus, meaning "pleasant place" in Latin.
 * All the rings of consciousness are named after common terms for the light zones in the ocean. The hadal zone, for example, refers to the deepest levels of the ocean, where there is little marine diversity, extreme pressure, and total darkness.