As we know that force is inversely proportional to area on which it is being applied.
now according to this law if the area increases the force decreases, when area decreases force increases.
now we know they on a nail bed there are about more than hundred nails. now due to increased no of nails area is also increased so the force decreases.
so the force applied on a person's body is very less than it looks. that's why a person can lie on a nails bed.