Mark A. W. Andrews, professor of physiology and director of the Independent Study Pathway at the Lake Erie College of Osteopathic Medicine, replies:
Generally speaking, heaviness of the muscles around the eyes, including the levator muscles that open the upper eyelids, is similar to fatigue of any muscle of the body. Ocular and brow muscles are especially prone to fatigue because they are active for most of our waking hours. Over the course of the day, they gradually grow leaden with extended use, as our arms and legs do.
Such a feeling may be compounded by general fatigue, including a lack of sleep, or by specific muscle overuse related to long hours of focusing on, say, a computer monitor. Excess skin of the eyelid, or prolapsed fat pads underneath the eyes, makes an individual more prone to this sensation. Chronic allergies and sinus infections may also exacerbate the heaviness, and sun exposure may cause eyelid swelling and thereby increase the probability that the drooping will interfere with vision.
Although heavy eyelids do not typically indicate underlying medical issues, some conditions do cause drooping eyelids, or ptosis. A stroke or a muscular disorder such as myasthenia gravis or myotonic dystrophy can damage facial muscles or their nerves and cause ptosis, as can elective facial surgery or interventions such as Botox injections to the brow.
Editor's Note: This story was originally printed in the November 2008 issue of Scientific American.