Science has shown that our biological rhythms through “a day” (i.e. “Nychthemeron” = “a day and a night) are regulated by a so-called “clock gene”.
 
B-persons have a clock gene lasting 25-27 hours, whereas A-persons have one that runs for approximately 23 hours.
    
Every individual has their own biological rhythm (known as the “Circadian rhythm”), which means we have our very own sense of time and planning of time. There are lots of different kinds of circadian rhythms. Most people are B-persons, which means that their circadian rhythm has a cycle that runs through 25 hours or more.

 B-persons have therefore a tendency to stretch their "nychthemeron”. A B-person finds it easy to stay awake in the evening; but has difficulties waking up in the morning – the time where the A-persons are the most active. Each human being has this so-called inner clock, which is regulated by the clock-gene.

A B-person stays awake until late at night – and goes to bed at 1 or 2 a.m. and sleeps a little longer in the morning.

If a B-person and an A-person go to sleep at the same time and rise early the same time next day, their mornings will be experienced differently, as the body-temperature of the A-person is higher than that of the B-person at this point. The B-person has therefore a very good reason for feeling drowsy, as it is still ”night” (”time for sleep”) for the B-person.

One family has been followed and tested for several years in the USA. They have "FASPS", familial advanced sleep phase syndrome. See, for example, http://www.hhmi.org/news/ptacek2.html. A group of scientists has found out that humans, who are extreme A-persons, have a genetic flaw or disturbance.

15 – 25 percent of the population is made of B-persons, 10 -15 percent are A-persons. The rest of the population are more or less either one or the other, or something in between.