Summarize the roles of homeostatic/allostatic/circadian factors, brain regions, flip-flop circuits, and neurotransmitter systems in regulating transitions between sleep and wake.

The homeostatic control of sleep sustains the balance of wake/sleep at an adequate level. Slep can also be monitored by allostatic control that reacts towards the environmental stressors to override homeostatic control when required.  

The circadian factors entrain periods of sleep to specific parts of the dark or light cycle. The slow-wave sleep takes place when the neurons in the vlPOA (ventrolateral peropic area) becomes active. These neurons prevent the neurons systems, which encourage wakefulness. In turn, the vlPOA is restricted by this similar wakefulness promoting regions, therefore, flip-flop circuit either keeps us asleep or awake.  

The gathering of adenosine encourages sleep by stimulating and preventing the sleep-promoting neurons of the vlPOA. The function of the orexinergic neurons of the lateral hypothalamus maintains the flip-flop, which monitors the waking and sleep state in the waking state.