Answer: Melanopsin is a light sensitive retinal protein that helps entrain the circadian rhythm.
Like other light sensitive transmembrane proteins found in the retina such as rhodopsin or photopsin, melanopsin changes its molecular configuration when activated by photons of light. Melanopsin in particular is highly sensitive to blue wavelengths of light (480 nm). About 2 percent of retinal photoreceptors are melanopsin containing.
A portion of the optic nerve axons project into the suprachiasmatic nucleus (SCN) via the retinohypothalamic tract. This tract sends electrical information from the melanopsin containing photoreceptors.
These signals into the SCN causes melatonin production by the pineal gland to be delayed. Melatonin is a sleep promoting neurohormone that increases drowsiness. Delaying melatonin production keeps you from falling asleep quickly.
Limiting blue light exposure before bedtime can help keep melatonin levels high at night, helping you get to sleep sooner. Try blue light blocking glasses at night to help you get to sleep better.