https://thenextweb.com/apple/2018/07/12/apples-face-id-thwarted-by-morning-face/

It is a truth universally acknowledged that the vast majority of the human population do not look their best first thing in the morning. And the iPhone X apparently agrees, as the phone’s signature Face ID feature reportedly struggles to recognize its users in their waking states.

An article on Slate tallies up some of the circumstances under which Face ID seems to flounder, and mentions that a common complaint is that Face ID won’t accept the face of a freshly-awoken person is that of its owner.