Light adaptation

It is something amazing our body does constantly without ever getting credit for it. The system we are referring to (and shall get due credit today) is our eyes ability to adapt to light and darkness.

As always, let’s go back to basics and run through the how the eye works. The eye lets in light via a varying sized hole termed the pupil. Once light enters, it travels through the eyeball until it hits the back which is filled with two types of receptors termed photoreceptors . You may have come across these receptors termed rods and cones. Both photoreceptors [rods and cones] contain a pigment (termed photopigment) which is made from vitamin A and a protein. When a particle of light hits the vitamin A it causes its structure to change shape. This shape change induces an electric charge which travels down the optic nerve before being relayed to various parts of your brain. The brain then interrupts the signal into an image.

Too little light and the photoreceptors fail to create the signal and too much light means that there are too many signals being created which overstimulates the brain. The consequence of this are all to clear when we talk about being ‘blinded by the light’. So how does the body regulate this? It’s very simply really but the beauty lies in the simplicity. The body controls;

  1. the amount of light entering your eyes, and
  2. the sensitivity of the nerves to the light.

Light entering your eyes stimulates the pupil reflex, an involuntary action on which the pupil size alters in relation to the amount of light. The brighter the light, the small the pupil will shrink thus making the inlet of light into the eye smaller. Therefore less light enters the eye.

In darkness the reverse occurs, because we want to capture as much light as possible. Once the light arrives, the optic nerve can ramp up/down its sensitivity (I.e. It’s acknowledgement ) of the signal created by the photopigments. This process however takes time as the nerve has to reorganise itself. Moving from a dark to bright [light adaptation] environment means the nerves downgrade their sensitivity to counteract the many signals created from the vitamin A changing shape. The process takes approximately 4-9mins. This is why we are initially blinded by the light but quickly recover our visual fields after a few minutes.

When moving the opposite way, light to dark [dark adaptation] our nerves become more sensitivity to accommodate the scarcity of signals. This process takes longer to adapt to, approximately 30mins. This explains why we fumble in the dark for so long when we wake up in the middle of the night!