Question

What does it mean that human beings can’t find their “off switch”

Answer 1

It most likely means that there mind cannot or won't shut off and they can't really go into an unconscious state(sleep) and stay awake.
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Answer 2

That can be taken many different ways maybe: they push themselves to hard and don’t know when to take a break or they cant settle down, sleep or, don’t know when to stop.