Ptosis due to aging or dehiscence of the levator aponeurosis: This condition is a common cause of eyelid droop, especially in older adults, due to the weakening of the muscles or tendons that lift the eyelid.

Horner's syndrome: Characterized by a triad of ptosis, miosis, and anhidrosis, although in some cases, the ptosis may be the only presenting symptom initially.
Third cranial nerve (oculomotor nerve) palsy: This can cause ptosis, along with other symptoms like diplopia and pupil dilation, but sometimes the ptosis can be an isolated finding.
Myasthenia gravis: An autoimmune disorder that can cause fluctuating muscle weakness, including eyelid droop, which may be more noticeable after periods of activity.

Cavernous sinus thrombosis or tumor: Although rare, these conditions can cause third cranial nerve palsy and require urgent medical attention due to their potential for serious complications.
Aneurysm (especially posterior communicating artery aneurysm): Can compress the third cranial nerve, leading to ptosis, and is a medical emergency if ruptured.
Stroke or cerebral vasculitis: In some cases, a stroke or cerebral vasculitis can present with isolated third nerve palsy, including ptosis.

Chronic progressive external ophthalmoplegia (CPEO): A mitochondrial disorder that leads to progressive weakness of the external eye muscles, including the levator palpebrae superioris, causing ptosis.
Oculopharyngeal muscular dystrophy: A rare genetic disorder that affects the muscles of the eyes and throat, leading to ptosis among other symptoms.
Botulism: A rare but serious illness caused by a toxin that can lead to muscle weakness, including eyelid droop, among its symptoms.