Every moment of our lives, we experience fluctuations in our temperature, blood pressure, brain waves, energy levels, attentiveness, appetite, hormone production, and much more. The beating of our hearts is one such rhythm; so is the menstrual cycle of women. The science of chronobiology is concerned with identifying our cycles and internal clocks and seeing how they interact. Although the hypothalamus and the pituitary gland are undoubtedly influential in keeping us on our own schedules, no one yet knows how they how they do it. We are, of course, also influenced by the outside world. Sunrise is nature's prod to wakefulness, and the alarm clock is a man-made aid. But our biological rhythms - with or without external cues - have a persistence and strength that scientists are just beginning to appreciate.
When a biological cycle is completed in a period of 24 hours or so, it is called circadian, from the Latin for "about a day." The daily pattern of wakefulness and sleep is a circadian rhythm. Sleep is often accompanied by fairly regular temperature changes. No two people have identical rhythms, but a common pattern is for the body temperature to be at its lowest between 4:00 A.M. and 6:00 A.M. By 7:00 A.M., the temperature may be almost at 98.6 degree F (37 degree C); by midmorning, when temperature is at its highest, energy is highest too.
So-called "morning" people, who rise up raring to go, generally start their temperature rise not at 5:00 A.M., but at 3:00 A.M. Although their energy peak occurs earlier, so does the falling off of their energy at the other end of the day.
The temperature of "night" people may not rise till 9:00 A.M., which means feeling listless while others are in high gear. Their peak performance may not come until late afternoon or early evening.