Do Snakes Have Eyelids? How Snakes Sleep With Open Eyes

The brille: a clear scale instead of eyelids

Look closely at a snake’s eye and you see a smooth, glassy surface with no lids around it. That surface is the brille, from the German word for “spectacles.” It is a single transparent scale, part of the outer skin, that sits over the front of the eye.

Scientists believe the brille evolved from eyelids that fused together and became transparent. Instead of moving over the eye, the “eyelids” became a permanent window. Between the brille and the eye itself is a narrow, fluid-filled space, so the eye can move slightly behind its cover.

The brille does several jobs that eyelids do in other animals:

• Protection. It shields the eye from dust, soil, sand and scratches, which is useful for animals that push through leaf litter and burrows.
• Moisture. It keeps the eye from drying out, so snakes do not need to blink to spread tears.
• Cleaning. When it becomes scratched or dirty, it is replaced at the next shed.

Why snakes lost their eyelids

The most widely accepted idea is that early snakes or their ancestors lived at least partly underground. Burrowing in soil exposes the eyes to grit, and a sealed, transparent cover is better protection than eyelids that open. Many burrowing lizards also have fused or transparent eyelids, which supports this idea.

The very simplest snakes, the blind snakes and thread snakes that live underground, go further still. Their tiny eyes lie under ordinary, partly see-through head scales and can probably detect little more than light and dark.

How do snakes sleep with their eyes open?

Snakes do sleep, but because they cannot close their eyes, it is hard to tell. Researchers and naturalists look for changes in behavior rather than closed eyes:

• The snake lies still for long periods, often tightly coiled, with its head resting on or tucked under its body.
• It stops flicking its tongue.
• It reacts more slowly to movement, light or vibration, and may take a moment to “wake up” when disturbed.

The pupil can still narrow in bright light, which reduces the light reaching the eye. Many snakes also choose dark, sheltered places such as burrows, crevices and hollow logs to rest, which helps keep light out. Sleep in reptiles is much less studied than sleep in mammals and birds, so how deeply snakes sleep and how much they need is still poorly known.

What happens to the brille when snakes shed?

Because the brille is part of the skin, it is shed along with every other scale. Before a shed, fluid forms between the old and new layers of skin, including under the brille. That is why a snake’s eyes look cloudy or milky blue for several days before it sheds. The fluid then clears, and when the snake crawls out of its old skin the old eye cap comes off with it.

If you find a complete shed skin, you can often see the two little clear domes where the eyes were. The whole process is explained in Why do snakes shed their skin?.

How good is snake eyesight?

Eyesight varies widely between species. Burrowing snakes see poorly, while some tree snakes and fast daytime hunters, such as the vine snakes of Asia and the Americas, have keen vision and can judge distance well. Many snakes see movement much better than fine detail.

Most snakes rely heavily on other senses. The corn snake, for example, has fairly ordinary eyesight and finds prey mainly by smell, using its forked tongue to collect scent particles. That system is described in Why do snakes flick their tongues?. Pit vipers, pythons and some boas can also sense the heat of warm-blooded prey.

Eyelids: a quick way to tell a snake from a legless lizard

Many lizards have lost their legs and look remarkably like snakes. Eyelids are one of the easiest clues to tell them apart.

The slow worm of Europe and the glass lizards of the eastern United States both blink, which instantly shows they are lizards. See Is a slow worm a snake? for more. A few lizards, including most geckos, also lack movable eyelids and have a spectacle like a snake’s. Many of them keep it clean by licking it with their tongue.