The eye has a third type of cell. And no one knew it existed.

Background

For decades, we've known that fully blind people — with no working cones or rods — still kept their circadian rhythms and still responded to light. That meant something else in the eye was picking up light. Scientists spent years trying to find out what it was.

What the study looked at

Berson's team identified and studied retinal ganglion cells that connect directly to the suprachiasmatic nucleus — the brain's master clock. They blocked every signal from cones and rods and checked whether these cells still responded to light on their own.

What they found

They did. The intrinsically photosensitive retinal ganglion cells — known today as iPRGC — fired in response to light even with no input at all from the classic visual photoreceptors. They're independent light sensors, with their own pigment — melanopsin — and their own direct line to the hypothalamus. Their job isn't to see. Their job is to tell the brain what time it is.

What this means for you

The system that controls your baby's sleep isn't the same one that lets them see your face. They're two separate systems. And that means a light can leave your baby's vision completely alone — it doesn't wake them, it doesn't dazzle them — while still switching on their body clock and wiping out their melatonin, with neither of you noticing. Until they've been awake for two hours and won't go back down. That's exactly why we built the NeuroSafe™ Light: one tap gives you the light you need without flipping that switch in your baby's brain.

Study limitations

The study was done in rats, not directly in humans. But the presence of iPRGC in humans was confirmed that same year by Hattar et al. and later by many human studies. The mechanism is the same.