Allergic Reaction: A sudden onset of dark eyelids could be due to an allergic reaction, which may cause increased blood flow and swelling, leading to a darker appearance. This is a common and relatively harmless condition.

Fatigue or Lack of Sleep: Prolonged periods without adequate rest can lead to increased blood flow and puffiness around the eyes, making the eyelids appear darker.
Dehydration: Inadequate hydration can cause the skin to appear dull and darker than usual, including the eyelids.
Hormonal Changes: Fluctuations in hormone levels, such as those experienced during menstruation or menopause, can affect skin pigmentation and lead to darker eyelids.
Sun Exposure: Increased melanin production due to sun exposure can cause the skin around the eyes, including the eyelids, to darken.

Thyroid Disease: Certain thyroid conditions, such as hyperthyroidism, can cause changes in skin pigmentation, including darker eyelids. Although less common, thyroid disease can have significant health implications if left untreated.
Adrenal Insufficiency: This rare condition can cause changes in skin pigmentation, including darkening of the eyelids, and is potentially life-threatening if not diagnosed and treated promptly.
Malignancy: In rare cases, darkening of the eyelids can be a symptom of a underlying malignancy, such as melanoma. Early detection is crucial for effective treatment.

Melasma: A condition characterized by patches of discoloration, typically on the face, which can include the eyelids. It is more common in women, especially during pregnancy.
Addison's Disease: A rare endocrine disorder that can cause changes in skin pigmentation, including darker eyelids, due to the deficiency of adrenal hormones.
Vitamin Deficiencies: Deficiencies in certain vitamins, such as vitamin B12, can cause changes in skin pigmentation, leading to darker eyelids. However, this is relatively rare and usually accompanied by other symptoms.