Here's what's actually happening in your legs at hour twelve.
When you stand all day, gravity wins.
Blood and fluid pool in your legs because the muscles that normally pump it back up — your calves, mostly — aren't getting the movement they need. Walking and bending act like a second heart for your lower body. Standing still does the opposite.
By the end of a long shift, your legs are full of fluid that has nowhere to go. That's the heaviness. That's why your shoes feel tighter at 7pm than they did at 7am.
Elevation helps a little. So does walking. But the fastest way to move that fluid is the same way your body does it naturally — rhythmic, sequential pressure from the foot upward. The exact thing your calves stopped doing six hours ago.