Going into hibernation is a gradual process. Over a period of days, the heart rate and breathing rate drop slowly, reaching slow steady rates of just a few times per minute. The body temperature also plummets from mammalian levels of 101.5–103.5°F (38.6–39.7°C) to 50–68°F (10–20°C). The lowered body temperature is regulated about the new set point, and therefore makes fewer demands on metabolism and food stores.
Electrical activity in the brain almost completely ceases during hibernation, although some areas remain active. These are primarily areas which respond to external stimuli such as light, temperature, and noise, so that the hibernating animal can be aroused under extreme conditions.