Most animals, especially those that enter prolonged hibernation, must prepare for this process — this preparation period is called photoperiodism.
Mammals sense the approach of sleep time due to changes in temperature. For warm-blooded animals, the signal is the onset of cold weather, while for inhabitants of deserts and tropics, it is a sharp increase in temperature and lack of rain. At such moments, animals begin to actively gain weight and stock up on food, as well as search for and prepare a shelter for hibernation. Only bats can afford to unexpectedly fall asleep and enter a 'crisis' short hibernation.
It is worth noting that many animals periodically wake up during hibernation, for example, to replenish the necessary substances in their bodies — to eat and drink water. Additionally, during hibernation, animals continue to monitor their condition. If the temperature in their shelter changes, or if it becomes too humid or dry, they may wake up and start looking for a new place for their seasonal 'rest.'
The only creature that can remain asleep during hibernation (unless it is awakened, of course) is the brown bear.
