A unit measuring the experience level of a dreamer, in addition to orbits. One sphere is equivalent to ten orbits.
Spheres form the basis for many restrictions in the city, most notably portals that are restricted only to higher sphere levels.
Once a dreamer reaches each ninth orbit, they must find a teacher that has the ability to train sphere levels, and is of a higher sphere level than the student. The student will be required to complete an arduous task before the teacher advances them to the tenth orbit.
The sphere equivalent in arts is referred to as a plateau. The same process applies.
Also see the XP and Sphere Table.