Black bear hibernation involves five phases: normal activity, excessive eating and drinking, fall transition, hibernation and walking hibernation. The duration and timing of phases depends on region and food availability.
Normal activity extends from spring until the middle of summer or autumn. Bears ideally consume between 5,000 and 8,000 calories a day during normal activity. An absence of food and water at this stage can prove fatal for a black bear later in the hibernation cycle. Around the middle of summer or fall, black bears enter hyperphagia, during which they double or triple their food consumption and water intake. The extra water is necessary to flush nitrogenous waste from the body after excessive food intake. A single bear can excrete several gallons of urine a day during hyperphagia. During fall transition, bears eat less but continue to drink. They rest most of the day, and their active heart rate drops from 80 to 100 beats per minute to 50 to 60 beats per minute. Sleeping heart rates may be as low as 20 beats per minute. During hibernation, black bears use 4,000 calories a day but do not eat, drink or excrete waste. Heart rates fall even lower and they take a breath every 45 seconds. When bears emerge from hibernation, they enter a stage called walking hibernation for up to three weeks. During this stage they eat and drink less than when in normal activity as their metabolism gradually returns to normal.