The primary factor in the physiology that underlies our early bird and night owl behaviours is the circadian rhythm of body temperature. Though we usually assume that body temperature is constant except when we are ill, it, in fact, cycles approximately 1o C every 24 hours. It is highest during the day and lowest at night when we are sleeping. Though such a small change seems insignificant, it does have a major effect on our daily behaviour.
Both early birds and night owls experience a body temperature high and low every 24 hours. However, early birds achieve their peak body temperature earlier in the day than night owls do. Given that we are most active and alert when our body temperature is highest, one can understand why early birds are more alert and active early in the day, while night owls do not become entirely alert and active until later in the day.
Added to this is the fact that, once they are awake, early birds experience a rapid increase in body temperature and then maintain somewhat of a plateau during the day with a somewhat smaller increase in temperature early in the evening. Body temperature in night owls, on the other hand, increases very gradually from the time of waking throughout the day until a peak is reached later in the evening. For this reason, night owls are usually very slow to get going in the morning, but as their temperature continues to rise throughout the day, they continue to become more and more active and alert.
Early birds reach their peak body temperature early in the evening and then experience a rapid decline. For this reason, they are not alert later in the evening and usually want to sleep. The body temperature in night owls, however, remains high, and does not peak until about an hour after the early birds have reached their peak body temperature. Night owls are, therefore, able to be alert and keep functioning much later into the night.
Throughout the day, the body temperature of night owls is generally lower than that of early birds, though the peak body temperature achieved by both groups tends to be about the same.