Why do animals Hibernate? Hibernation is a type of deep sleep an animal enters. This helps to keep its energy needs to the minimum and helps them regulate their body temperatures better. As they are sleeping, they do not need to search for food, and the fat and calorie build up helps them get through their hibernation period. There are different habits followed by animals that hibernate. Like a squirrel sleeps for about 4 to 5 days at a stretch and will wake up, again eat something of the food it has collected all summer. Go for a toilet break and them go back to sleep for 4 to 5 days. When it comes to bears, they just sleep all winter, without a break and wake up fresh and very hungry after winter is long gone. Animals know when winter is about to start with the help of external stimuli. When they do know it is time to hibernate, they produce a hormone called 'hibernation-specific protein'. This hormone starts lowering their metabolic activity and induces sleep state. Soon, the heart rate drops, breathing becomes slow and the blood supply gets restricted. The non-essential organs start entering a state of stasis. This leads to drop in core temperature and the internal temperature is more or less equal to external temperature. Thus, the energy requirement is reduced and the animal can survive winter. When you have a look at the list, you will find a variety of species who follow this survival strategy. Mammals such as bats, bears, ground squirrels, undergo hibernation. Reptiles like tortoises and snakes, too undergo sleep state. Birds like Poor-Wills, swifts, and nighthawks, as well as amphibians like toads and frogs too hibernate. These animals store food as body fat during the months of summer and fall. When they hibernate they use up their body fat for the little energy that they need to live during hibernation. Some animals like squirrels store their food in burrows and caves. This food works as a light snack when they do wake up from their sleep for a break. In case of cold-blooded animals, their body temperatures drop with the drop in environmental temperatures. This means their body temperatures will match the environmental temperatures. When the temperatures outside begin to change, and the air becomes warmer, these animals will wake up from their deep sleep.