From Brain Pickings by Maria Popova:
But in Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired, a fine addition to these 7 essential books on time, German chronobiologist Till Roenneberg demonstrates through a wealth of research that our sleep patterns have little to do with laziness and other such scorned character flaws, and everything to do with biology.
In fact, each of us possesses a different chronotype — an internal timing type best defined by your midpoint of sleep, or midsleep, which you can calculate by dividing your average sleep duration by two and adding the resulting [corresponding?] number to your average bedtime on free days, meaning days when your sleep and waking times are not dictated by the demands of your work or school schedule. For instance, if you go to bed at 11 P.M. and wake up at 7 A.M., add four hours to 11pm and you get 3 A.M. as your midsleep. [I go to bed about 10pm and awake about 5am on weekdays (= 1.5), and weekends are relatively the same (perhaps = 2.0). So, I think my Chronotype is 3.5. What is it?]
Internal Time goes on to illuminate many other aspects of how chronotypes and social jet lag impact our daily lives, from birth and suicide rates to when we borrow books from the library to why older men marry younger women, and even why innovators and entrepreneurs tend to have later chronotypes. (One hypothesis: because they were more challenged in school than early types, and always had to invent clever strategies to help them perform despite not being on top of things.)