Is it normal that i see small objects move when i'm tired?

Let me explain with a example.

Let's say there is a penny in my bedroom floor. I see the penny and when I turn my attention to something else, out of the corner of my eye I can see the penny dart away. But when I look at what I thought had moved (the penny) it's still in the same pace.

Also I can see like a small floating black dot darting off from the corner of my eye but when I look to see if it's a fly or something there's nothing there.

Is this normal?
(Pardon any typos, I'm on mobile.)