What Causes Blurry Vision At Night?

Having a harder time seeing things at night? You are not alone. There are actually legitimate reasons why it becomes harder to see at nighttime.

Increase in Prescription

The first thing that happens when lights get dim is that our pupil dilates. This is actually advantageous! By dilating, the eye absorbs more light to make out details a little better. However, this dilation also can cause a small change in your prescription. To understand why this is the case, let’s look at how the cornea is shaped.

The cornea isn’t a perfect sphere. When you travel from the center of the cornea outwards, the cornea progressively gets flatter. Extra light traveling through the flatter sides of the cornea get focused in a different direction than light traveling through the front of the cornea. Normally the undilated iris blocks this light, but when the iris dilates at nighttime, this light makes it way back to the retina. This causes something known as spherical aberration. This can actually cause the prescription to shift leading to a term known as night myopia (or night nearsightedness).

So what can be done about this? Well, an easy adjustment is to just bake just a little extra nearsighted prescription in glasses, contact lenses or vision correction surgery. Thus, when the eye dilates, the prescription still works out. Want another solution? Turn on a little bit more ambient light. This causes the pupil to constrict a little and eliminate that extra night myopia.

But night myopia isn’t the only reason why vision can blurry at nighttime.

The Travel of Colors

Night myopia doesn’t cause everything. There exists something called “chromatic aberration.” But don’t fear! The concept isn’t as difficult as the name lets on.

What is the color of light? Well, light can exist in different colors! All the colors of the rainbow in fact! Let’s take a tangent and talk about rainbows for a sec. When you take white light and shine it through a prism (which is like a lens shaped like a triangle), it spreads the light out into the different colors. This occurs because red light exists on a longer wavelength and bends less through the prism than blue light.

_{Rainbow, image by Jim Evans, CC BY-SA 4.0, via Wikimedia Commons}

So what do rainbows have to do with nighttime? Well, turns out that the same thing that forms rainbows also happens in all lenses including our eye (but to a much smaller degree). Red light isn’t focused at the same spot at blue light in our eye. We have a mini-rainbow that passes through our retina with the red light focusing slightly behind our retina and the blue light focusing slightly in front of it. Normally, this isn’t much of an issue since during daytime, yellow light (dead in the middle of the spectrum) is most common and thus everything is focused pretty well.

However at nighttime, this changes. Instead of having white light everywhere, shorter wavelengths of light such as the blues and the violets become more common. Thus, the blue lights at nighttime become slightly out of focus on your retina. This equals some additional blurriness at nighttime!

Adjustment of Retina

So we have a slight change in prescription and a differences in the travel of colors, but there is more to the story…

Let’s start off by learning about rods and cones. Inside the back of our eye is the retina. The retina takes all the light information from what we are looking at and sends that information to the brain. The two types of cells in the retina which capture light information are called rods and cones (named for their shape on a microscopic level).

• Cone cells are responsible for doing all the heavy lifting for most of our vision. These cells excel in recognizing colors and are densely populated in the center of the retina, the macula, to recognize fine details. However, cones aren’t perfect; cones aren’t specialized at picking up low levels of light.
• This is where rods enter the picture. Rods are kinda like the reverse of cones. Rods don’t do color. Rods aren’t used to recognize fine details. Such a bummer, except for the fact, that rods are excellent at picking up light! Rods allow us to see very faint light sources such as stars in the sky.

_{Stars at Night, image by Bureau  of Land Management, Public domain, via Wikimedia Commons}

But switching between primarily using cones to primarily using rods doesn’t just happen automatically. It takes some time. This process is called dark adaptation and in humans takes about 20-30 minutes after exposure to bright lights before our eyes are finely tuned to see the dim lights at nighttime. Therefore, if you just walked outdoors from a brightly lit room, this is another reason why it can be difficult to see at night. At least until the eyes adjust your new environment.

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