Dark to Light reponse: increase in cortisol. This is why it is best to wake up after is is already light
Light to dark response: decrease in cortisol, which is why suddenly switching off lights does not work to lower cortisol.
Knowing exactly how your circadian rhythm works is the secret to getting more energy while sleeping less. Circadian rhythm is the key process in your body that controls how long you sleep! When you read "The Polyphasic Sleep Mastery" you will learn exactly how this process works, and how you can start doing simple things during the day that will immediately throw your circadian rhythm back into its natural powerful state.