How to make the bottom box shadow stretch across the whole header

I recently found out how to have the box shadow show up on only one side of an element, however, it doesn’t go the full length of the element, and stops a bit short of the edge of the screen on both sides as shown in the code snippet and image below. Does anyone have a fix to this/is it even possible to make it stretch the whole way across? All help is appreciated.

Example Image

header {
    box-shadow: 0 5px 7.5px -7.5px gray;
<header>Header Shadow Example</header>