Because blue light exists naturally in abundance from the sun, our biology is primed to interpret blue light as daytime– time to be awake! This mechanism works through, you guessed it, our eyes. The melanopsin receptor in our eyes send a signal to the suprachiasmatic nucleus, which in turn activates or deactivates the pineal gland, which produces melatonin.
Melatonin, the hormone that reduces our alertness and invites sleep. Although melatonin is produced by various tissues in the body the major source is the pineal gland. Melatonin produced by the pineal gland occurs in the absence of light. Although, to some extent, our body will attempt to retain the circadian rhythm, the pineal gland will cease melatonin production after even one second exposure to blue light wavelengths.