9 (B) The Circadian Rhythm and the Homeostatic system.

Our body has a Circadian Rhythm. Our circadian rhythm acts as a 24-hour internal clock. Our Circadian Rhythm is regulated by the Suprachiasmatic Nucleus.  A Suprachiasmatic Nucleus is a group of nuclei located in the hypothalamus.  The Suprachiasmatic Nucleus expresses clock proteins.  The function of these clock proteins set the pace for the day of activity it controls sleep and hormone releases. 
                                       
The Homeostatic system responds to longer wake periods by adding an urge to sleep.  Neuroscientist discovered that people who are awake have an increased level of adenosine.  The level of adenosine increases during sleep.  Adenosine also decreases during cellular activity.