There are a few reasons why people close their eyes when they sleep. For one, it helps to block out any external stimuli that could potentially disturb your sleep. When your eyes are closed, you are less likely to be disturbed by outside noise or light. Additionally, closing your eyes can help you to focus on your internal sleep environment. This can help you to relax and fall asleep more easily. Finally, closing your eyes can help to prevent you from accidentally falling asleep in an uncomfortable position.