Hibernation is used for conserving energy by some animals, and when they hibernate, bears enter a deep sleep. Given these facts, why do bears enter a deep sleep among the following options:
- Thyroid-stimulating hormone
 - prolactin release
 - chemical messengers
 - They can't live in the cold
 - To get some rest
 - To conserve energy
 - Endocrine system
 - They don't like the winter


 

      To conserve energy   