The Retinohypothalamic Tract (RHT) is a neural tract that goes from the retina all the way to the suprachiasmatic nuclei (SCN) via the optic nerve and optic chiasm
Suprachiasmatic Nucleus
The SCN is the primary internal biological clock/pacemaker of the body
Retinal cells called intrinsically photosensitive retinal ganglion cells (IpRGC) contain a pigment called melanopsin, which is sensitive to bright light. Upon exposure to light, it communicates electric signals via the optic nerve to the SCN that informs the brain to remain awake
This is responsible for waking us up and keeping us awake during bright light exposure. Indeed, bright light alters the circadian rhythms to ensure our continued wakefulness and that’s why you should try not to look at bright screens during bedtime and you should be exposed to bright natural light during the morning