New Delhi: People who stay up late and wake up late may be more intelligent than those who prefer early nights and mornings, according to a 2009 study published in Personality and Individual Differences. Researchers Satoshi Kanazawa and Kaja Perina analysed data from the National Longitudinal Study of Adolescent Health (Add Health) and found that higher childhood IQ was associated with greater nocturnality in early adulthood.
The researchers examined a large, nationally representative sample of young Americans. The Add Health study initially interviewed 20,745 adolescents in 1994–95. By 2001–02, 15,197 of the original participants, then aged 18 to 28, were interviewed again. The researchers used this later group to examine their sleeping habits alongside their childhood IQ scores.
The findings showed a consistent relationship between childhood intelligence and later sleeping patterns. People with higher childhood IQ scores tended to go to bed later on weeknights and weekends and wake up later on weekdays. The researchers found a significant association between childhood IQ and three of the four measures of nocturnality they examined.
The difference was visible even across the study’s IQ categories. Participants classified as “very dull”, with childhood IQ scores below 75, went to bed at around 11:41 pm on weeknights in early adulthood. Those classified as “very bright”, with IQ scores above 125, went to bed at around 12:29 am. The researchers also found that the very bright group tended to wake later, particularly on weekends.
The association remained after the researchers accounted for a range of factors that could influence sleeping patterns, including age, sex, race, marital and parental status, education, earnings, religion, whether participants were students and the number of hours they worked each week.
The study’s explanation for the finding comes from what the researchers called the “Savanna–IQ Interaction Hypothesis”. They argued that nocturnal behaviour is relatively evolutionarily novel because sustained activity at night was uncommon in traditional ancestral environments. The hypothesis therefore predicts that individuals with greater intelligence would be more likely to adopt such evolutionarily novel preferences. The researchers used circadian rhythms as one example of a preference that can be influenced by both biological predisposition and individual choice.
Also read: Winner-takes-all Wimbledon-style payoffs await India’s white-collar workers
Based on verbal intelligence
The researchers were careful about the strength of the relationship. Although the association between childhood IQ and nocturnality was statistically significant, they noted that the effect size was small, as was the amount of variation in sleeping patterns explained by IQ. They also acknowledged that the study measured verbal intelligence rather than general intelligence directly and relied on participants’ reported sleeping habits rather than direct behavioural measurements.
So, the study did not establish that staying up late makes someone more intelligent. Rather, it found that people who had higher childhood IQ scores were, on average, more likely to become night owls as young adults. The researchers concluded that their findings supported their hypothesis that more intelligent individuals are more likely to adopt nocturnal behaviour.