Well explained...and I like the way you used the thermostat analogy to explain dosing with the circadian rythyms.  It made it very easy to understand.  You could even go as far as to say that each day would be different from the next in a normal healthy person.  Some days the thermostat would be working very hard and you would be using heaps of fuel and other days, it would hardly be needed.  What I ean to say is some days the usage is normal, some days it is excessive.  Just like the need for cortisol differs from one hour to the next or from one day to the next day dependant on the stresses etc.  It is a good way to explain to a child as Laura said.