The term Ptosis is used to describe the drooping of the upper eyelids. Those people who have ptosis usually have trouble opening their eyes completely. This covering of the upper eyelids can cause tiring of the eyes and reduces the field of vision. Ptosis is caused by a weakened levator or Müller's muscle, which is the main eye opening muscle. When compensating, people suffering from ptosis often unconsciously lift the eyebrows or forehead in the effort of raising the drooping eyelids.