Early to be, early to rise makes a man healthy, wealthy, and wise - Benjamin Franklin
There is a romance about all those who are abroad in the dark hours - Robert Louis Stevenson
Researchers have long recognized that many biological processes correspond to a (more or less) 24-hour cycle.
These circadian rhythms (from the Latin circa meaning about and diem meaning day), have been widely observed in humans, plants, animals, insects, and even single-celled organisms. Examples include our natural eating and sleeping patterns, hormoneproduction, brain wave changes, and even leaf movements in many plants.
While these rhythms are endogenous, i.e., determined by our basic biology, external cues such as sunlight, temperature, and availability of food can often be used to "reset" the body's internal clock through a process known as entrainment. This is what allows us to overcome jet lag whenever we travel across time zones and our bodies adapt to new sleeping and waking schedules.
This entrainment process can also lead to enormous individual variations in terms of our basic sleep-wake patterns and the time of day during which we are most active. Usually referred to as circadian preferences, these variations seem to be relatively fixed in most people and twin studies suggest that genetics play a strong role in how these preferences develop.
The most common examples are those people who have a morning preference, i.e. early risers who are most mentally and physically active during the morning hours, and the "night owls" who are more alert at night and prefer to sleep in most mornings. There are even psychometric tests that have been developed to measure these preferences including the Morningness Eveningness Questionnaire (MEQ), the Composite Circadian Scale, and the Composite Scale of Morningness.
To read more, check out my new Psychology Today blog post.