Our body clocks are tiny clusters of nerve cells in the center of our brain, about the size and shape of a letter "V" on this page. Like the thermostat connected to our home furnace, our body clocks make our body temperature and blood pressure rise before we usually awake in the morning. Moreover, it triggers the release of "get up and go" hormones we need to start the day. Evidently, our body clocks make us predictably alert in the daytime, sleepy at night, and hungry when we habitually eat. Besides, "it controls cell division, bone growth, and other basic body maintenance". Therefore, the tick and tock of our body clocks hold numerous implications for our life (Smolensky and Lamberg, ch.1). One more very important thing about our body clocks is that everybody has an individual profile or chronotype that describes our rhythmic behavior over twenty four hours, and, what is more, the profile will vary from individual to individual.
Here the scientists found out that humans could be categorized as falling into several chronotypes, and physiologist Nathaniel Kleitman was the first to separate the main of them, larks ("morning types") and owls ("evening types"), differing from each other as the sun and the moon (Alward 1337). The differences in "lark and owl traits influence many aspects of daily life, including when we feel most alert and when we sleep best". Besides, these different "traits determine when we most enjoy meals, exercise", work, and other activities (Smolensky and Lamberg, ch.5). So, it's easy to see that larks and owls seem to be completely different.
First and foremost, these two groups of people differ when taking into account our physiological characteristics. For example, the highest body temperature for larks is around 3:30 p.m. and the lowest one is around 3:30 a.m.,