Mark A. W. Andrews, professor of physiology and director of the Independent Study Pathway at the Lake Erie College of Osteopathic Medicine, has explained that the 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.
And especially, ocular and brow muscles are prone to fatigue because they are active for most of our waking hours. During a normal day, these muscles gradually grow leaden with extended use, just like our arms and legs.
Such a feeling is further enhanced by general fatigue, including a lack of sleep, or by specific muscle overuse related to long hours of focusing on, say, a computer monitor.
A person becomes more prone to this sensation because of the excess skin of the eyelid, or prolapsed fat pads underneath the eyes.
The heaviness is further compounded by chronic allergies and sinus infections, reports Scientific American.
Also, sun exposure may cause eyelid swelling and thus increases the probability that the drooping will interfere with vision.
While heavy eyelids do not typically indicate underlying medical issues, but there are some conditions that 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, just like elective facial surgery or interventions such as Botox injections damage the brow.