Why does the pitch of my voice fluctuate depending on these circumstances?

It seems when I don’t get much sleep, or no sleep at all, or don’t rest well that my voice gets deeper. Also, it seems that when I’m nervous or having a stressful first day of work that it gets higher. And not just during the time that I’m nervous, but it kind of sticks in that high register for a day or two and I can’t get it back to normal. What makes this happen? It’s frustrating when I’m trying to sing to my favorite jams and I can’t. :)