Hibernating bats are highly vulnerable to disturbance in winter when they cannot feed to replace energy lost to arousals from torpor.
Disturbing hibernating bats causes a welfare harm entirely disproportionate to the apparent severity of the disturbance. A brief human intrusion causes the bat to arouse from torpor, spending 30 days worth of fat reserves in one event. Repeated disturbance across winter causes starvation as fat reserves are exhausted before spring. This is why bat hibernation sites are some of the most stringently protected wildlife areas in UK law.