Hibernation is used for conserving energy by some animals.
When they hibernate, bears enter a deep sleep.
Bears enter a deep sleep to conserve energy.
Why do bears enter a deep sleep?

  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