Why is it that a person can lie down on a bed of nails unharmed?

A bed of several nails are more comfortable than one single nail.Ask the person to stand one or two or just not more than 3 pins to make show!

Pressure is the weight force  divided  by the area on which the body rests. The more the area of the weight force is spread over , you feel more comfotable.

Pressure per unit area reduces if the weight force is distributed over a larger area, larger number of pins.

On the other hand you consider the reaction, you get an equal reaction from  the area on which the weight force acts by Newton's 3rd law..

The reaction or is extremely high if the man is  made to lie or stand on a single one or  two sharp nails. If the average man wieighs 70 kg , his weight force of 700 Newtons shall be reacted back from from one or two nails or needles , he will be thoroughly punctured from one side to through the body to the other side.

That is the reason a number of nails do not make pain.