You're tired. You want to sleep. Or rather, you want to be asleep.
To go to sleep is a bit harder.
You need to walk through the kitchen to go and brush your teeth, go to the toilet.
Make sure that you don't make a mess on that short trip.
But knowing that you're not to do that, that is inevitably the only thing you can do.
The creature you imagine while washing your face, the black shadow you see when you look down, it isn't there. But because of the sole reason that you made him up in your head, has made him be. Now, go to sleep and know that he isn't because he's just imaginary.
But, since he's just an imaginary being, why is he so very much alive?