Eyelid droop (also called ptosis) is an abnormally low position (drooping) of the upper eyelid and occurs when the muscles that raise the eyelid (levator and Muller's muscles) are not strong enough to do so properly. It can affect one eye or both eyes and is more common in the elderly, as muscles in the eyelids may begin to deteriorate. The drooping may also be worse after being awake longer, when the eyelid muscles are tired. If severe enough and left untreated, the drooping eyelid can cause other conditions, such as amblyopia or astigmatism. Ptosis can also be congenital.