If I brush my teeth (+ all other things in preparation such as locking the door, feeding the cats, etc) and go to bed, then read (without disturbing my partner) then I am ready to fall asleep as soon as I feel tired.

However, if I stay up without brushing my teeth (+ the other preps) then I cannot go to sleep right away when I'm tired. Because I still got errands to do.

When I read in the first situation I am already lying in bed horizontally. In the second situation I am sitting. Also, while you're reading, you should use an e-reader without much blue light.