Your atlas spins on your axis. That is how rotation works in the upper cervical spine — C1, the atlas, is a ring that pivots around the dens, the odontoid process, of C2, the axis. About half of all the rotation your head performs happens at this single joint.
You have not used most of it in years.
The desk gives you two destinations to rotate toward — the second monitor, around thirty degrees, and the coffee mug, around twenty. For everywhere else, you turn your trunk. The atlanto-axial joint, the most rotation-rich joint in the entire body, gets used inside a 30° window when its capacity is closer to 90°. The cartilage thins. The capsule contracts. The rotators on either side stop knowing how to fire in the unused range.
Then the practice asks for full head rotation in Marichyasana C and D, in Pasasanamaybe not in Sirsasana entries. The head does not have it. The shoulder hikes to compensate. The trunk wrenches. The bind feels punishing.
The drill: sit tall. Eyes lead. Slowly turn the head toward one side until the shoulder wants to rise — that is the cap of pure cervical rotation. Try to gain another degree without recruiting the shoulder. Five reps each side, watching for asymmetry. The screen-side has more range. The coffee-side has less. That gap is the work.