How Light Pollution Prevents Eyes from Adapting to the Dark

Guest post from Tony Pistilli, board member of Fountain Hills Dark Sky Association

Why are Dark Skies Important?

On the surface of this subject, it appears non-Dark Sky compliant lights scatter light into the sky and prevent us from “seeing” stars. This is partially true. But an underlying cause is the effect that light has on our eyes and brains as we navigate a dark night.

It Takes Time for the Eye to Adapt

All plant, animal and human life have a biological clock that depends on light and darkness for health. As we become more dependent on lights in our businesses, homes, and streets, the desire for more lighting continues to grow, and many cities are as bright at night as they are during the day.  There are reports that animals, who are much more susceptible to light due to limited brain function, become confused and disoriented when bright city or suburb lights cause migrating birds to crash into skyscrapers.

Similarly, human eyes take several hours to fully adapt to darkness and reach their optimal sensitivity to low light conditions. The quickest gains in vision sensitivity are made in the first few minutes after exposure to darkness. For this reason, many people think that after only a few minutes, their eyes have reached their peak sensitivity. But several hours into darkness exposure, the human eyes continue to adapt and make small gains in sensitivity.  With the brightness of city and town lights, our eyes never become “dark adapted”.

The Biological Process of Dark Adaptation

Rhodopsinis the photopigment used by the rods in our eyes and is the key to night vision. Intense light causes these pigments to decompose reducing sensitivity to dim light.  Darkness causes the molecules to regenerate in a process called “dark adaptation” in which the eye adjusts to see in the low lighting conditions.

The problem is that rhodopsin is so sensitive to light that under normal light levels, this chemical is deformed and deactivated (called “photobleaching”). Most of the day, when we are walking around in normal light, the rhodopsin in our eyes is deactivated. Upon exposure to darkness, the rhodopsin is able to regenerate and reactivate, becoming sensitive again to light and improving our night vision. But this regeneration process takes time. Cone cells take about 10 minutes to adapt to the dark. Rod cells take quite a bit longer, up to 10 hours.

Lastly, the rod cells in our eyes are responsible for black and white vision. They are the heavy-hitters when it comes to vision in low light conditions. The rods in our eyes achieve this great night vision through several mechanisms:

  1. Like cones, rod cells contain rhodopsin, the chemical that is highly sensitive to light. In fact, rod cells rely more heavily on rhodopsin than cone cells, leading each single rod cell to be about 100 to 1000 times as sensitive as a single cone cell once fully adapted.
  2. There are far more rods on the retina (100 million) than there are cones (5 million).
  3. Several rods all connect to the same output signal (the same interneuron). This fact allows lower levels of light to be detected at the cost of image resolution.
  4. Rods respond slowly to light (they collect light over long time periods). This slow response means that lower levels of light can be detected at the cost of sensing rapid changes in time.

Take Time to Adapt and Support Dark Sky Preservation

Cone cells take several hours to become completely dark adapted. Expert naked-eye astronomers know this fact well. They will give their eyes several hours to adapt instead of a few minutes in order to maximize their vision of the dim stars. In summary, upon exposure to darkness, our pupils dilate in a matter of seconds, our cones adapt in 10 minutes, and our rods adapt completely after several hours.