Everyone expects some degree of reduced visibility at night — but what makes it night blindness? In order to recognise symptoms of nyctalopia for yourself, it's important to first understand how our eyes work in low light conditions, and how some common eye conditions can impact our night vision in different ways.
How do we see in the dark?
Our ability to see well in the dark relies on four key parts of our eye:
- The cornea – This is a clear tissue that sits over the pupil, at the front of the eye. Light passes through the cornea to enter the pupil. Some conditions can cause the cornea to change its shape or become less clear and this can impact the quality of light that enters the eye. Where the cornea is not clear or distorted, it’s harder to create a clear image.
- The pupil – That’s the black ‘hole’ in your eye which is opened and closed by the iris (the coloured ring around it), depending on how much light is available. When it's dark, the pupil gets bigger (dilates) to allow as much light as possible to enter the eye and reach the retina. In bright light, the pupil gets smaller (constricts), allowing only enough light to see properly.
- The lens – Once light enters the eye, it passes through the lens, which focuses light onto the retina. When there is low light, it makes it harder to create a clear image.
- The retina – This is the 'screen' to your pupil 'projector' onto which images are shone. It consists of rods and cones that absorb light and convert it into images to send to the brain. The rods are great at absorbing and processing light, even when it’s dark. The cones are responsible for helping us see in detail and colour, but they need a lot of light to work well, which explains why we don’t distinguish colours or detail well in low light.
If an underlying eye condition affects one or more of these parts, it can make it even more difficult to see in dark conditions, leading to night blindness.
What causes night blindness (nyctalopia)?
Night blindness is a catch-all term for various eye conditions that make it very difficult to see at night or in low light environments. Here are some of the more common causes:
Glaucoma develops when there is damage to the optic nerve and the nerve fibres from the retina, this is usually because of high pressure within the eye.
Because glaucoma affects the sensitivity of vision in general, night vision also becomes an issue. Both day and night are affected as nerve fibres become damaged.
Glaucoma medications that constrict the pupil can also make it harder to see in the dark because ordinarily the pupil would be wide to let in as much light as possible in.
Cataracts occur when cloudy patches develop in the clear lens inside your eyes, stopping light from reaching the back of the eye and causing blurred or cloudy vision. As a result, people with cataracts might find it especially difficult to see clearly in low light conditions, and they may see blurry halos around bright lights (such as street lamps or car headlights) when driving at night.
Retinitis pigmentosa (also known as RP) develops as a result of certain genetic disorders which cause the breakdown of cells in the retina. Once these photoreceptor cells in the retina begin to degenerate, people with RP will notice a gradual decline in their vision, such as struggling to see at night and losing their peripheral vision.
In most cases, it takes our eyes about 20 minutes to fully adapt to low-light conditions, but with RP, it can take much longer or not happen at all. Eventually you may be diagnosed with night blindness which will likely restrict your ability to drive to the daytime only.
If you have diabetes, you’re at risk of developing an eye condition known as diabetic retinopathy, a complication of diabetes that damages the light-sensitive retina. As with many other retinal conditions, any damage to the retina would usually make it harder to see at night.
Keratoconus occurs when the cornea (the round, clear front part of the eye) starts to weaken and thin at its centre, causing it to become irregular (or coned) in shape. Eventually this prevents the eye from focusing properly, which causes poor vision. This poor vision will be exacerbated in low light. Any damage or distortion to the cornea would usually make it harder to see at night.
Night blindness symptoms
Everyone will find it harder to see detail in low light levels, but if you have night blindness, you might also experience the following symptoms, depending on the underlying condition that’s causing it:
- Difficulty focusing in dim light
- Blurry or cloudy vision
- Discomfort when moving from a bright environment to a darker one
- Poor peripheral vision
- Difficulty seeing into the distance
- Problems adapting to sudden bright lights (for example, car headlights of street lights) which can make driving at night difficult
- Headaches and nausea
- Sensitivity to light (also known as photophobia)
If you are experiencing any of these symptoms, it’s important to book an eye test as night blindness can often be caused by an underlying eye condition.
How is night blindness detected?
Night blindness is a symptom of a range of existing eye conditions, so your optometrist can perform a range of eye tests to help discover the underlying cause.
These tests will, for example:
- Check the pressure in your eyes (a key indicator of glaucoma)
- Assess your ability to focus at distance to see if you’re short-sighted, long-sighted or have astigmatism
- Check the health of vital structures in your eyes
Some examples of tests that your optometrist might perform include:
- Eye pressure test (tonometry): An instrument called a tonometer is used to measure the pressure inside your eye (intraocular pressure). Tonometry can be useful to identify ocular hypertension, which is a risk factor for chronic open-angle glaucoma.
- Autorefractor: To see how long or short-sighted you are we use an autorefractor. These tests can be performed on separate pieces of equipment or on a single multifunctional machine.
- Snellen test: To fine-tune their findings, the optometrist will ask you to read a test chart that measures your visual acuity, i.e. how well you can see with and without lenses in front of your eyes. This can help to determine whether you may have difficulty seeing at night because you need to wear glasses.
- Visual field test: This assesses the sensitivity of the central and peripheral vision — a common symptom of night blindness. You will be shown a sequence of light spots and asked which ones you can see. Some dots will appear in your peripheral vision, which is where glaucoma begins. If you can’t see the spots in your peripheral vision, it may indicate the glaucoma or retinitis pigmentosa has damaged your vision.
- OCT (Optical Coherence Tomography): OCT scans can help our opticians to detect signs of eye conditions up to four years earlier than traditional imaging methods. They produce a 3D-like image that allows us to see the structures of your eye in even greater detail, and help us to manage the development of conditions (such as glaucoma and diabetic retinopathy) over time.
You can learn more about our tests on our eye tests explained page.
How is night blindness managed and treated?
Treatment for night blindness will depend on the cause. Remember, night blindness is a catch-all term for various eye conditions that make it very difficult to see at night or in low light environments so any management or treatment will be linked to these underlying causes.
Your optometrist will explain all your options and answer any questions you might have.