In neuroanatomy, the retinohypothalamic tract (RHT) is a photic neural input pathway involved in the circadian rhythms of mammals.

[a][2] The suprachiasmatic nuclei receive and interpret information on environmental light, dark and day length, important in the entrainment of the "body clock".

Studies done on rats show that even with severely degenerated photoreceptors (blind, no visible light perception), they have the ability to entrain to the light/dark cycle because they have intact RHTs.

[5] Further development of the brains of those in the NE group showed that the two suprachaismatic nuclei (SCN) have nearly equal inputs shortly after the pathway is cut.

This was shown to drastically slow down the re-synchronization of internal biological rhythms to the external time cues, primarily light.

Rats in the AE and NE groups similarly reduced the amount of fluid intake during the study during the hours they were exposed to constant light.