Dayworld is a science fiction novel by American writer Philip José Farmer.

Published in 1985, it is the first in the Dayworld tetralogy of novels inspired by Farmer's own 1971 short story "The Sliced-Crosswise Only-On-Tuesday World".

The story is set in a dystopian future in which an overpopulated world solves the problem by allocating people only one day per week.

Jeff is caught and put in a sort of insane asylum, classified with “multiple personality disorder”, for the legal time before he can be considered “incurable” and killed.

But Jeff has an escape plan... Dave Langford reviewed Dayworld for White Dwarf #84, and stated that "another fast-moving example of a far-out social setup within which the author merely plays cops and robbers.