The rhythm is linked to the lightdark cycle. Animals, including humans, kept in total darkness for extended periods eventually function with a free-running rhythm. Their sleep cycle is pushed back or forward each "day", depending on whether their "day", their endogenous period, is shorter or longer than 24 hours. The environmental cues that reset the rhythms each day are called zeitgebers (from the German, "time-givers"). Totally blind subterranean mammals (e.g., blind mole rat Spalax sp.) are able to maintain their endogenous clocks in the apparent absence of external stimuli. Although they lack image-forming eyes, their photoreceptors (which detect light) are still functional; they do surface periodically as well.[page needed]
To what is the circadian rhythm tied?
lightdark cycle

In the 20072008 school year, there were 181,973 undergraduate students, 20,014 graduate students, and 4,395 first-professional degree students enrolled in Oklahoma colleges. Of these students, 18,892 received a bachelor's degree, 5,386 received a master's degree, and 462 received a first professional degree. This means the state of Oklahoma produces an average of 38,278 degree-holders per completions component (i.e. July 1, 2007 June 30, 2008). National average is 68,322 total degrees awarded per completions component.
How many undergrad students were in Oklahoma in 2007?
181,973

According to the 2009 American Community Survey, White Americans of both Hispanic and non-Hispanic origin represented over one-fifth (22.9%) of the Bronx's population. However, non-Hispanic whites formed under one-eighth (12.1%) of the population, down from 34.4% in 1980. Out of all five boroughs, the Bronx has the lowest number and percentage of white residents. 320,640 whites called the Bronx home, of which 168,570 were non-Hispanic whites. The majority of the non-Hispanic European American population is of Italian and Irish descent. People of Italian descent numbered over 55,000 individuals and made up 3.9% of the population. People of Irish descent numbered over 43,500 individuals and made up 3.1% of the population. German Americans and Polish Americans made up 1.4% and 0.8% of the population respectively.
How much of the Bronx was non-Hispanic white as of 2009?
34.4%