Answer:
The force of the weight
Step-by-step explanation:
When we are lying on the bed, our weight is distributed along of our body and the equivalent of the force is given by the following expression.
w = m*g
where:
m = mass [kg]
g = gravity acceleration [m/s^2]
w = weight of the body [N]
At the moment when we are sleeping our body is at equilibrium, because the mattress is exerting a force with the same amount of the weight but in the opposite direction.