AdvertisementPerceiving without seeing: How light resets your internal clockDecember 17, 2022Ari DanielfacebookEmailFred Crittenden, 73, lost his sight to retinitis pigmentosa when he was 35 years old. Today he has no visual perception of light. "It's total darkness," he says. Still, he has cells in his eyes that use light to keep his internal clock ticking along nicely. (Marta Iwanek for NPR)