Our vision is often much better in bright, well-lit conditions than in dim conditions, however, even at night vision should be adequate for tasks such as driving, walking, and identifying known objects. There are multiple eye conditions which reduce night vision only or affect night vision before normal light vision.
Differences Between Dark and Light Conditions
In the dark, there are certain changes which will occur naturally to attempt to improve the vision in the absence of light.
The eyes will naturally dilate, meaning that the pupils get larger to allow more light inside the eye. In the dark, there is less light overall, and a larger pupil allows the eye to capture enough light to produce vision.
Additionally, the main photoreceptors at night and in the dark are rods instead of cones which are the main photoreceptors for the day.
Rod photoreceptors are more sensitive to small amounts of light and help to determine shapes and sizes in the dark.
Problems with Night Vision
Anything that reduces vision during the day can also impair vision at night. However, there are conditions which only affect night vision or affect night vision more than daytime vision.
Some inherited conditions including retinitis pigmentosa can cause early onset night blindness or nyctalopia.
Cataracts are a major cause of reduced night vision due to an increase in glare and halos around lights.
Even small amounts of astigmatism can become much more problematic at night due to the change in the pupil size.
Retinitis Pigmentosa and Nyctalopia
Retinitis pigmentosa is the most common of a group of inherited retinal conditions which all can lead to night blindness.
These conditions may cause slightly different changes in the eyes, but will all affect the rod photoreceptors exclusively or affect the rod photoreceptors before the cone receptors.
Retinitis pigmentosa, in particular, is frequently the cause of reduced night vision in young adults or teenagers as the age of onset for retinitis pigmentosa is around twenty.
In addition to the loss of vision at night, there may be constriction of the visual field in the periphery associated with retinitis pigmentosa.
There is currently not a cure available for retinitis pigmentosa, but vitamin A supplements, genetic testing, and low vision aids may be helpful in reducing the complications of the condition.
Cataracts: Glare and Halos
Cataracts are the most common cause of reduced vision in the elderly population. But even before the cataracts begin to affect the daily vision, there may be impacts on the night vision.
Cataracts are cloudiness in the crystalline lens inside the eye. This cloudiness can cause light to scatter when it enters the eyes.
This phenomenon is particularly noticeable and problematic at night when there is less light available in the environment.
Car headlights and streetlights may appear to have halos around them or there may be glare that is so intense that it affects vision for several minutes even after the light has passed.
If cataracts are causing impaired night vision, it may be worth having a consultation about cataract surgery.
Astigmatism: Irregular or Stretched Lights
Astigmatism is a type of refractive error, similar to nearsightedness or farsightedness. In astigmatism, the eye focuses light in two places instead of just one.
Since there are two points focused, the image (a single point originally) will be stretched into a line.
Small amounts of astigmatism may not require glasses or contact lenses as the vision is not severely decreased.
However, at night the pupil becomes larger and allows more peripheral light into the eyes.
If astigmatism is present, it may become artificially more at night with a larger pupil.
A common complaint with astigmatism at night is that lights look stretched or smeared into a line.
If this is the case, then glasses or contact lenses may be needed for nighttime wear.