Luke is very awake here, no third eyelid in sight.
Third eye lid? What the heck? Many people have no idea that dogs have additional eyelids. The haw or nictating membrane is found in the inner lower corner of the eye. All dogs have them, some are more visible than others. Often the droopy eyed breeds have much more obvious third eyelids. The third eyelid is a protection lid. You will see it appear as your dog starts to fall asleep. Some dogs will even sleep with their eyes open, giving them a very creepy appearance as the third eyelid covers over the eyeball. Tilley is an open eye sleeper.
Getting sleepy, third eyelid will be coming up soon.
Here you can see the lid making an appearance. It is almost covering Luke's entire eye. You can only see the very top of his eyeball which is not covered.
Here the lid is covering the eyeball almost completely. Luke's is very translucent giving him a creepy look as his eyeball rolls back when he falls asleep.
The third eyelid is also used to protect the eyeball in case of injury. It will cover over the ball and protect it from further damage. If your dogs third eyelid goes up and doesn't come down, then they may have an eye injury or a painful eyeball. Unfortunately the third eyelid can be damaged itself and there are many issues surrounding the third lid. Cherry eye is probably the most common third eyelid issue.
Problems with the third eyelid
Dogs that have very prominent third eyelids can have a sleepy sort of look. The lid itself can sit up quite high on the eyeball making it much more obvious. If the lid is a light color it can be even more obvious. Many people panic when they first notice the eyelid and believe something to be severly wrong with their dog.
The lid can come up for many reasons, sleep, injury, pain and even fever. So if you've never noticed it, watch your dog as they fall asleep. As with everything they are all different so it may be barely visible or very visible, but it's there.