Final answer:
The pressure in the air mattress likely decreased overnight due to cooler temperatures causing the air within to contract and the materials of the mattress to slightly increase in volume, leading to a flatter appearance in the morning.
Step-by-step explanation:
When considering the situation where one of the campers notices that his air mattress is flatter in the morning than it was the night before, several factors could contribute to the change in pressure. Despite the absence of holes, temperature changes can affect pressure. Overnight temperatures typically drop, which can cause the air inside the mattress to cool down.
According to the ideal gas law (PV=nRT), if a gas's temperature decreases while the volume remains constant, its pressure will also decrease. This is compounded by the fact that the materials of the air mattress may contract slightly when cold, effectively allowing the volume to increase marginally, which would further lower the pressure. Without adding or removing air, the pressure would decrease due to the drop in temperature.