Night Life Is Evolutionarily Novel - Why Night Owls Are More Intelligent than Morning Larks

The Intelligence Paradox: Why the Intelligent Choice Isn't Always the Smart One - Satoshi Kanazawa 2012

Night Life Is Evolutionarily Novel
Why Night Owls Are More Intelligent than Morning Larks

Virtually all species in nature, from single-cell organisms to mammals, including humans, exhibit a daily cycle of activity called the circadian rhythm. “This timekeeping system, or biological ’clock,’ allows the organism to anticipate and prepare for the changes in the physical environment that are associated with day and night, thereby ensuring that the organism will ’do the right thing’ at the right time of the day.”8 The circadian rhythm in mammals is regulated by two clusters of nerve cells called the suprachiasmatic nuclei (SCN) in the anterior hypothalamus.9

Geneticists have by now identified a set of genes that regulate the SCN and thus the circadian rhythm among mammals.10 A behavior genetic study of 977 South Korean twin pairs shows that the heritability in morningness-eveningness (whether you are a morning person or a night person) is .45 and nonshared environment accounts for 55% of the variance, while shared environment does not appear to explain any of the variance.11 So the circadian rhythm appears to be yet another human trait that roughly follows the 50—0—50 rule I discuss in Chapter 3.

“For most animals, the timing of sleep and wakefulness under natural conditions is in synchrony with the circadian control of the sleep cycle and all other circadian-controlled rhythms. Humans, however, have the unique ability to cognitively override their internal biological clock and its rhythmic outputs.12 While there are some individual differences in their circadian rhythm, where some individuals are more nocturnal than others, humans are basically a diurnal (day-living) species, as are all extant monkey and ape species, except for one.13

Humans rely very heavily on vision for navigation but, unlike genuinely nocturnal species, cannot see in the dark or under little lighting. Our ancestors did not have artificial lighting during the night until the domestication of fire. Any human in the ancestral environment up and about during the night would have been at risk of predation by nocturnal predators. It is therefore safe to assume that our ancestors rose at around dawn and went to sleep at around dusk, to take full advantage of the natural light provided by the sun, and that “night life” (sustained and organized activities at night after dark) is probably evolutionarily novel.

In order to ascertain the extent to which our ancestors might have engaged in nocturnal activities, I have once again consulted ethnographic records of traditional societies throughout the world. In the 10-volume compendium The Encyclopedia of World Cultures,14 which extensively describes all human cultures known to anthropology, there is no mention of nocturnal activities in any of the traditional cultures. There are no entries in the index for “nocturnal,” “night,” “evening,” “dark(ness),” or “all-night.” The few references to the “moon” are all religious in nature, as in “moon deity,” “Mother Moon (deity),” and “moon worship.” The only exception is the “night courting,” which is a socially approved custom of premarital sex observed among the Danes and the Finns, which are entirely western cultures far outside of the ancestral environment.

In addition, I have consulted the same five extensive (monograph-length) ethnographies that I consult in Chapters 5 and 6.15 Many of these ethnographies contain a section where the authors describe what usually happens and what people routinely do in a typical day in the tribal society under study.

These detailed ethnographic records make it clear that the day for people in these traditional societies begins shortly before sun- rise, and ends shortly after sunset. “Daily activities begin early in a Yanomamö village.”16 “The day begins about 6 a.m., when the sun is about to rise.”17 The only routine activities conducted after dark are people conversing and visiting with each other as they drift off to sleep. “Despite the inevitable last-minute visiting, things are usually quiet in the village by the time it is dark.”18 “Most evenings are spent quietly chatting with family members indoors. If the moon is full then it is possible to see almost as well as during the day, and people take advantage of the light by staying up late and socializing a great deal.”19 “After cooking and consuming food, evening is often the time of singing and joking. Eventually band members drift off to sleep, with one or two nuclear families around each fire.”20 The only nocturnal activities, other than chatting visiting, and making speeches, that I can find in all of these ethnographies is when Mukogodo men go searching for missing animals in the dark, if one happens to be missing.21

It may be significant in this context that humans mostly evolved in sub-Saharan Africa near the Equator, where the length of the day remains more or less constant throughout the year. So the practice of “wake up at dawn, go to sleep at dusk” would have produced days of roughly the same length throughout the year. At higher latitude, however, the same practice would have produced days of varying lengths, longer in the summer and shorter in the winter. In the extreme cases, near the arctic circles, days would never have ended in the summer and would never have begun in the winter. Thus humans in higher latitudes would have had to wake up before dawn and stay up after dusk if they wanted to have days of roughly the same length throughout the year. Incidentally, the average intelligence of populations tends to be higher at higher latitudes (and longitudes), even controlling for the average temperature.22

Ethnographic evidence of traditional societies therefore suggests that our ancestors probably had a largely diurnal lifestyle, and sustained and routine nocturnal activities may be evolutionarily novel. The Intelligence Paradox would therefore predict that more intelligent individuals are more likely to be nocturnal, getting up later in the morning and going to bed later in the evening, than less intelligent individuals.

Previous to my 2009 article with Kaja Perina,23 there had only been one study which examined the association between intelligence and circadian rhythm.24 The 1999 study found that, in a small sample of US Air Force recruits, evening types were significantly more intelligent than morning types. This is consistent with the prediction of the Intelligence Paradox.

The analysis of the Add Health data confirm this prediction of the Intelligence Paradox. Net of age, sex, race, marital status, parental status, education, income, religion, whether currently in school, and the number of hours worked, more intelligent children are more likely to grow up to be nocturnal in their early adulthood. More intelligent individuals go to bed later, both on weeknights and on weekend nights, and they wake up later on weekdays (but not on weekends, for which the positive effect of general intelligence on nocturnality is not statistically significant).

Figures 8.1 through 8.4 show that the association between childhood IQ and adult nocturnality is monotonically positive, even though absolute differences are not very large. For example, on weeknights, “very dull” children (with IQs below 75) on average go to bed at 23:41 in early adulthood, whereas “very bright” children (with IQs above 125) on average go to bed at 00:29. In general, the more intelligent they are in junior high and high school, the later they go to bed and the later they wake up in early adulthood. The probability that one would get the patterns as strong as those represented in the four figures above purely by chance, when there is actually no association between childhood intelligence and adult circadian rhythm, is one in 10,000 or smaller.

Figure 8.1 Association between childhood intelligence and weekday time to go to bed


Figure 8.2 Association between childhood intelligence and weekend time to go to bed


Figure 8.3 Association between childhood intelligence and weekday time to wake up


Figure 8.4 Association between childhood intelligence and weekend time to wake up