We left off talking about the circadian rhythm and how it is primarily regulated by light during the day, helping us be more alert, and conversely by melatonin at night, making us feel sleepy.  This can actually help define what type of person you are- either a lark (those who are always “bright-eyed and bushy-tailed” first thing in the morning), or an owl (people who “do their best work after 10 PM”).  It also goes along way toward explaining a major difference between teenagers and...