Fact 1: Hibernation is used for conserving energy by some animals.

Fact 2: When they hibernate, bears enter a deep sleep.

Given the two facts above, answer the question "Why do bears enter a deep sleep?" with 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   