This is due to the fact that there is little food (flying insects) for them to eat. If they stayed awake in a warm home, without hibernating, then they would starve. We can begin the bat exclusion process during the winter months.
Mean free path of molecules in a gas. The molecules in a gas are assumed to be in continuous random motion making collisions with each other and with the walls of the container. The mean distance that the molecules travel between one collision and the next is called the mean free path of the molecules.
The Lindhard function of free electron gas is so elementary that its closed form can be obtained in clarity. Despite its simplicity, some nontrivial physics manifest. In 1D, the divergence of static response function ($\omega=0$) indicates the well known Peierls instability and the system tends to break its translational symmetry.