Out of sheer exhaustion? Does it defintely means "get exhausted?"

Rex:
Of course, if I don’t get some sleep pretty soon,
I’ll be forced to move back upstairs out of sheer exhaustion. (he goes upstairs)

I don't get clear about the meaning of "out of sheer." Please let me know lol

Question

Out of sheer exhaustion? Does it defintely means "get exhausted?"

Rex: 
Of course, if I don’t get some sleep pretty soon, 
I’ll be forced to move back upstairs out of sheer exhaustion. (he goes upstairs)

I don't get clear about the meaning of "out of sheer." Please let me know lol

Answer

Out of = because of. It tells you the cause of something (in this context).
I made a lot of mistakes out of carelessness.
She told my boyfriend I'd cheated on him out of malice.
So, he will be forced to move back upstairs because of sheer exhaustion.