Their occurrence is coordinated by an area within the anterior hypothalamus, the suprachiasmatic nucleus, and disorders related to these in humans include advanced sleep phase syndrome.  
In mammals, these entities may be influenced by light-sensitive melanopsin ganglion cells. They are entrained by a periodic environmental signal called the zeitgeber and are called "free running" in the absence of such a signal. Their occurrence is coordinated by an area within the anterior hypothalamus, the suprachiasmatic nucleus, and disorders related to these in humans include advanced sleep phase syndrome.  In diurnal animals these are controlled by levels of melatonin. For 10 points, identify these cycles providing the body with an internal clock lasting approximately 24.3 hours in humans. 
In mammals, these entities may be influenced by light-sensitive melanopsin ganglion cells.
circadian rhythms 