A heavy head feeling is not a diagnosis by itself. But in craniocervical instability, it can be part of the pattern.

The skull and upper spine depend on ligaments, joints, and muscles working together. In people with connective tissue disorders, ligament laxity can affect the craniocervical junction, where the skull meets the upper cervical spine.

When the craniocervical junction is not providing enough support, the head can begin to feel too heavy for the neck to comfortably support, especially after being upright, sitting at a screen, riding in a car, or holding the head still for too long. The muscles may tighten in response, but the sensation itself is often deeper than ordinary muscle fatigue.

Because there is no standard test for a “heavy head” feeling, patients are often left trying to describe a biomechanical problem in language that sounds subjective.

"My head feels too heavy" was not a strange thing to say. It was a clue.

The 2017 review on neurological and spinal manifestations of the Ehlers-Danlos syndromes (link) discusses how ligament laxity at the craniocervical junction can contribute to neurological and spinal symptoms in connective tissue disorders.