Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: In some locations, squirrels sleep for long periods of time during the winter months. Which of the following most likely causes these squirrels to sleep for long periods of time?
decrease in temperature
Hibernation.