As far as I remember, there is nothing to suggest that the inhabitants of this mythical land were particularly sleepy, but then again the book of Genesis is hardly notable for the accuracy or believability of the stories that it contains.
Of course this legendary area to the East of the Garden of Eden is where Cain was sent to after he had murdered his brother Abel. (Genesis 4:16)
In fact, it seems likely that the people who were sent to Nod had a lot of hard work in front of them. Having been banned form the Garden of Eden they had to start to work hard to survive.
Maybe that is why they were tired?