Night vision is the ability to see in the dark, or dim environment. This is the opposite of daytime vision which is the ability to see in bright light. Vision is made possible by a part of the eye called the retina. This retina is located at the back of the eye and it is responsible for seeing both during the day and at night. The retina is made up of specialized nerve cells called photoreceptors that receive and process light energy and relay the processed message to the brain through the optic nerve. The retina contains two categories of photoreceptors: rods and cones.. Rods are used for night time or dark vision and cones help to see in the daytime. There are 20 times more rods than cones in the eye. This means the cells that help you see in the dark are much more abundant than those that help you see in the daytime. These rods contain photopigments called rhodopsin which consists of a protein and the retinol form of vitamin A.
How do we see in the dark?
Your eye and brain work together to see at any time. To see in the dark, light travels through the cornea, the anterior chamber of the eye, the pupil (a hole in the centre of the iris), the lens, and the posterior chamber before striking the rod cells in the retina. These rod photoreceptors convert the light energy into neural impulses, which travel down through the optic nerve to the vision processing areas in the brain.
Cornea and Anterior Chamber: the cornea is the transparent dome at the front part of the eye. It refracts light upon entering the eye. The anterior chamber is a pool of fluid behind the cornea. The cornea and the anterior chamber must be clear and transparent for light to pass through.
Pupil: It's the aperture located at the centre of the iris. When light hits the pupil it contracts to reduce the amount of light entering the retina. In darkness, the pupil widens to let in more light to enable good night vision.
Lens and Posterior Chamber: the lens is a transparent organ in the centre of the eye. The posterior chamber is another fluid filled body in the eye, behind the lens and in front of the retina. These two must remain clear and transparent for light to pass through to get to the retina.
Retina: when light hits the retina in the dark, the rod photoreceptors undergo a chemical reaction that converts that light energy into electrical impulse that travels through the optic nerve to the brain to be interpreted as what you see.
Can your eyes see in total darkness?
No, your eyes cant see in total darkness. Your eyes’ rod cells aren’t so sensitive that they can make something out of nothing. Your night vision is good outdoors and in your home because it’s almost never completely dark. Light from various sources such as the moon, stars, streetlights, or alarm clocks is always present, providing just enough illumination for your eyes to function. Eye diseases and hereditary conditions that affect the retina and especially the rod cells will impair vision in the dark, cause poor night vision or even total night blindness.
Nyctalopia also known as night blindness, refers to difficulty seeing in dim light or at night like in a restaurant or movie theatre. Daytime vision, however, is unimpaired. Nyctalopia is usually a symptom of a problem in the retina. Certain eye diseases that can impair night vision or cause total night blindness include:
Vitamin A deficiency
Vitamin A is necessary for normal visual function. The rod cells in the retina contain photopigments called rhodopsin which consists of a protein and the retinol form of vitamin A. Nyctalopia associated with vitamin A deficiency is reversible and managed with vitamin A supplementation. Vitamin A deficiency can happen if your diet is lacking in vitamin A or if you can’t absorb the nutrients you need. In addition to your eyes, vitamin A deficiency can affect your skin, heart, lungs, tissues and immune system.
Retinitis Pigmentosa
RP is the most common inherited eye disease. It generally affects both eyes. Most people with RP have low vision. Some do go blind. Vision loss usually starts from the periphery of the eyes where the rod cells are most concentrated and therefore, poor night vision is usually the first and most common symptom.
High Myopia
Myopia, or nearsightedness can cause nyctalopia. It’s common for people who have myopia to have some difficulty with night vision. The blurry vision associated with myopia may be accentuated in dim light, manifesting as a common cause of nyctalopia. Corrective lenses and prescription eyeglasses improve dim light vision. This is not caused by retinal disease but by optical problems.
Others include glaucoma, age-related macular degeneration, and retinal detachments.
Most causes of night blindness are preventable. Don’t forget, if you find that it’s getting harder to see in dim light, or you have a harder time adjusting to light changes, you should talk to your optometrist.