They don't have the same once-per-24-hour sleep cycle as humans.

And Vizlet has grown very skilled at being able to tell if a yinglet is lying; if one of them claimed to be a human-turned-yinglet, they almost certainly wouldn't include vital little details like sleep patterns, among other things.

And, while yinglets require less overall sleep, they exhaust their energy supplies quicker than most species as well, so they will take short power-naps anywhere from 3 to 7 times in one 24-hour period. More on that later.