Keeping track of time when you cannot see light

Fred Crittenden is a 73-year-old man. Despite being blind and not able to see light, he is able to wake up every morning around 6:30 without an alarm.

This mystery was solved by Iggy Provencio, a biologist at the University of Virginia:

👁 Rods and cones are the cells of our eyes that allow us to see light. However, he discovered a molecule called melanopsin in frogs and mice that is responsible for detecting light but it is not in the rods and cones.

☀ Melanopsin is instead found in large neurons called melanopsin cells, which are parked in a different layer of the retina. These cells are sensitive to bright, blue light, which is found in the sun and in our phones, tablets, screens, and some indoor and outdoor lights.

🧠 The tentacles of the melanopsin cells reach out and touch around 30 brain regions, including the suprachiasmatic nucleus, which is the master circadian clock in our brain.

⏰ The suprachiasmatic nucleus uses the light information fed to it by the melanopsin cells to tell the rest of our body when it is time to sleep and when it is time to wake up.

Mice with their melanopsin switched off could see light, but their sleep patterns were off. They were given a lab-mouse version of jet lag, where the lights were suddenly shifted. These mice took a month to reset to the new time zone instead of just seven days.

This variability in the system is why some people have a harder time adjusting to daylight saving time or a change in time zones than others.

Therefore, it is likely that Fred Crittenden’s internal clock is still influenced by the light he is exposed to, even though he cannot see it.

Learn more about the effects of light on our health and how to balance our internal clock with light here.