This section is from the book "The Hygienic System: Fasting And Sun Bathing", by Herbert M. Shelton. Also available from Amazon: The Hygienic System Vol III Fasting and Sun Bathing.
In general the time of the initiation of hibernation corresponds closely with the scarcity of food and lowering of temperature. The termination coincides with the return of favorable conditions. Some species, or some individuals, however, may commence the hibernating period while factors are still quite favorable, or may terminate the period at an unfavorable time. Modern theories of the mechanism stress the physiological sequence of events characteristic of the process. These events may apparently be set into activity under any one of several external conditions.
In temperate climates bears eat more, especially of flesh in the Fall, as they are laying up a store of food in preparation for their winter hibernation. They literally gorge themselves on foods which they convert into fat, but when they enter the dormancy period, stomach and intestines are empty.
Hibernating animals may be induced to awaken readily by "strong external conditions." Following awakening, there is gradual elevation of body temperature and a regaining of normal physiological activity and behaviour. Lowering the temperature of the body to approximately 0° C. (32° F.) has been reported to awaken hibernating mammmals, though some investigators report that animals may often be killed by freezing without awakening.
Just as there are some migratory birds that do not return home until May and leave again in August, so some hibernating animals do not come out of their dark quarters for as many as seven months. Their hibernating period is one of complete fasting. In general, the period of hibernation corresponds with the period of cold and food scarcity.