You know when your mom calls and tells you to get more sleep because you sound exhausted? And you're all, "Moooom!" Well actually she probably knows her shit: researchers say our voices change in perceptible ways as we get tired. According to ABC:

"Police" sounded more like "Bolice." Higher energy letters such as T, P and K sounded more like D, B and G, respectively.

"Just the simple letter D tends to change as a person gets tired," said Frederick Brown, researcher and associate professor of psychology at Pennsylvania State University. "And how soon or how delayed they are when try to say it seems to be different than when they're wide awake."

So get some sleep already, or when you say "cat" it will sound like "gat" and everyone will be afraid of you.