Why can't we sleep with open eyes?

Actually some people CAN sleep with their eyes open!! :) I had a classmate who could. The only way we knew she was asleep was when her head would nod off! But, her eyes remained open... lol... lucky for her!!
Oh yes!! My baby does that too! I just hope he doesn't use that to sleep during his classes later in life!! :)

Some people sleep with open eyes but cannot see anytrhing during sleep. This is really a question for students of biology.

You cannot sleep with open eyes for the same reason you cannot sneeze with open eyes. It's human biology. It is an evolutionary trait.

To enter in the world of dreams ............you need to get out of real world......and closing eyes is the only option.

Firstly human eyes are designed such that it is impossible to sleep with open eyes, some people do sleep with eyes open but it is due to conditions such as Bell's palsy or other rare eye diseases. While sleeping, our brain goes to rest and is supposed to not respond to any stimuli of light, movement etc. which would not be possible if one slept with open eyes. So humans HAVE to sleep with eyes shut!

At the very outset let me define Sleep in very common language. Sleep is unawareness of surroundings.
A healthy person’s sleep is that condition where the mind comes to a rest including all the organs of the body. That means it includes eyelids also. But, the mind will be working.
One can sleep with eyes open only who has sleep disorders. Such people are called Parasomniacs. Parasomniacs are set of sleep disorders. They are sleep walking, sleep sex, Rapid eye movement, Sleep paralysis to name a few. Let’s take the case of a sleep walker. Some sleepwalkers walk in sleep with eyes open but oblivious of the external happenings. Because practically he is sleeping with eyes open.

Sandhya If you want to make sure if someone is asleep, you will have to check the eyes of the person to see if they are closed. Teeth grinding, sleep walking, open eyes, wet wetting are sleep related diseases and human can't sleep with their eyes open unless they have a disease. You know there are some animals which sleep with their eyes open and even in standing position.

There are few people who sleep even opening their eyes. You may not come across such persons but i saw

The body expects the environment to be dark while we try to sleep and hence closing the eyes is an integral part.