Check that pin! https://www.pinterest.com/pin/860680178768438763/ What a fun project to do
I don't think that would be sturdy enough for an adult, you'd be better off using a painter's canvas thats heavy duty and less like to rip with you in it. If you plan on hiking or camping with it, they are usually made from rip-stop nylon..... https://www.youtube.com/watch?v=X3cOiC0Bitg&t=17s
I would think you would have to double up on the sheets to make a hammock.
Sheets are not made to hold the weight of a human. Possibly if you sew several together you might get away with it, but that is why they are made of canvas or rope.
This is such a cool idea. I found this post. https://homesteady.com/12002590/how-to-make-your-own-hammock-from-bedsheets