The idea that society will one day split into Morlocks who shun the light and Eloi who like to bask in it, popularised by H G Wells in The Time Machine, may not be so far off the mark.

At a discussion of the social implications of our 24/7 society, held at the Science Museum's Dana Centre tomorrow, Prof Till Roenneberg of Ludwig Maximilians University, Munich, will describe how he has discovered that people have a "chronotype" which influences their health and profession.
—Roger Highfield, "Researchers shed some light on owls and larks of the workplace," The Daily Telegraph, March 29, 2004

Humans have been defined in terms of three major circadian types or chronotypes, using measures such as the Horne-Ostberg Morningness-Eveningness Scale, a subjective instrument which has been correlated with body temperature and other physiological factors. The three types are "morning," "evening" and "indifferent" or "mid-range". The first two categories each represent approximately 15% to 20% of the human population and the "indifferent" or "mid-range" category applies to the majority (60% to 70%) of humans.

A morning-type individual, or "lark", is defined as one whose circadian rhythms are skewed about two hours (or more) earlier than the norm for the human population as a whole. That is, larks naturally awaken between 4:00 a.m. and 6:00 a.m. and are ready for sleep by 8:00 p.m. to 10:00 p.m. To a lark, midnight is perceived as the middle of the night. Conversely, an evening-type individual, or "owl", is defined as one whose circadian rhythms are skewed about two hours (or more) later than the norm for the population. That is, owls naturally awaken between 8:00 a.m. and 10:00 a.m. and do not find themselves feeling sleepy until the midnight-to-2:00 a.m. time frame.
—"Chronotype Test," Round-the-Clock Systems, 1999

Evening people" drink more alcohol and coffee and smoke more than "morning people," Spanish researchers found.

A sample of 537 subjects aged 21 to 30 completed questionnaires on personality, preference for time of day and consumption habits.

They were classified according to "chronotype" (morning, evening or neither).
—"'Evening people' drink, smoke more," Journal of the Addiction Research Foundation, August, 1994