It starts fine. By mid-afternoon there is a band of tightness across the top of the shoulders, a dull ache at the base of the skull, and the specific urge to roll your head around and hear something crack.
Most people call this bad posture and feel vaguely guilty about it. That framing is not very useful, because it suggests the fix is willpower.
It is a load problem, not a character flaw
Your head weighs roughly ten to twelve pounds. When it sits balanced over your shoulders, the muscles of your neck barely work to hold it there. Skeleton does most of the job.
Move that head forward a couple of inches, which is what happens when you lean toward a screen, and the muscles have to start actively holding it up. Now a group of muscles designed for short bursts of positioning work is doing continuous load-bearing, for hours, every day.
They do not fail dramatically. They just get tired, then dense, then sore, then they start referring pain into your head.
Why it shows up in the afternoon
Muscles can hold a position for a while before they complain. The morning is usually fine because the tissue is fresh and you are still moving between meetings, coffee, and conversations.
By the afternoon you have accumulated hours of sustained low-level contraction without much variation. The muscles have not had the chance to lengthen and shorten the way they are built to. That is when the ache arrives, and it is also why standing up and walking for two minutes genuinely helps more than it sounds like it should.
The muscles involved
Three areas usually carry it. The upper trapezius across the top of the shoulders, the levator scapulae running from the neck down to the shoulder blade, and the suboccipitals, a small group at the base of the skull.
That last group is small but it punches above its weight. The suboccipitals sit right where the skull meets the spine, and when they stay tight they are a common source of the headaches that start at the back of the head and wrap forward behind the eye.
What treatment does
Hands-on work breaks the holding pattern. Sustained, specific pressure through those muscles restores length and rotation, and most people notice they can turn their head further before anything pulls.
We also look at the front. Chest and shoulder tissue that has shortened from reaching forward all day will quietly pull you back into the same position, so releasing the back without addressing the front tends not to hold.
What actually helps between sessions
The honest answer is variation, not a perfect ergonomic setup. The best chair in the world still has you in one position.
Stand up every half hour or so, even briefly. Raise your screen so you are not looking down at a laptop. If you take calls, take some of them walking. None of this is dramatic, and all of it does more than buying another cushion.
If the headaches are frequent, or if the ache has gone from an afternoon thing to an all-day thing, that is usually the point where it stops resolving on its own and some hands-on work is worth it.