题干
“Body clocks” are biological methods of controlling body activities. Every living thing has one. In humans, a body clock controls normal periods of sleeping and waking. It controls the times when you are most likely to feel pain. Eating, sleeping and exercising at about the same time each day will help keep body activities normal. But changes in your life—a new job, for example—destroy the balance and thus cause health problems. You could have difficult sleeping, for example. Scientists suggest that early afternoon is the best time of day for physical exercise. They say we are stronger and faster then. Later, about four o'clock, most people feel tired because of the drop of sugar level in the blood. Early afternoon also is a good time to do tasks about memory（记忆） and mathematics. If you must study, however, do it just before going to sleep at night. Scientists say you are more likely to remember information when there is a short delay（耽搁）between study and bed. Body clocks control all the activities from day to day. They also produce changes about every ninety minutes during the day. At the end of such a time period, our ability to think hard begins to become weak. That is the time we are most likely to day-dream. Within fifteen minutes, we are ready to go back to what we were doing.
