Day by Night by Tanith Lee

This repackaged edition of a classic sci-fi tale from a master storyteller explores a planet of great contrasts, one side in perpetual light, the other in darkness.
Vel Thaidis is a figment of Vitra's imagination. In a city with no sunlight, Vitra crafts dreams to entertain the masses. She enjoys a decadent life with the nobility while the lower class work and rot.
Vitra's dreams are a mirror image of her life. Vel has a brother like her, knows a man like the one Vitra desires. Even the machines that take care of them, that no one remembers how to fix, are the same. Except in Vitra's dreams, no one can fix the machines when they slowly die, while they never break down in reality. Vitra will never fear being stuck in the dark with no machines to create light.
Until she is. Vitra's dreams and reality are merging. She feels pushed into a corner, with no solution but the one her dreams have given her. In the end, Vitra may not have as much free will as she believes—and her dreams may be more real than she knows.