People show a sense of time. Some people have a better sense of time than others. Do you know people who can plan to awake at let’s say six o’clock in the morning and do so? Such people awake fifteen minutes earlier or later, but no more. Scientists who have studied this ability believe people have a sort of built-in clock in them, which marks time. Each person uses a day-night cycle of sleep and wakefulness that repeats day after day. A person’s temperature falls a very little at bedtime and perhaps a little more at night. When a person gets up in the morning his temperature climbs back to normal. In some individuals the temperature climbs quickly and they are ready to work efficiently soon after they get up in the morning. In others the temperature rises slowly. These people are more efficient later in the day. So we can classify people as day people and night people. Day people are happy at breakfast time and feel they are ready to work. Night people often feel unhappy after t