Saturday, November 7, 2015

The missing hour of sleep

Rêverie, Adrien de Witte (1850-1935), Wikicommons

African Americans sleep on average almost an hour less than do Euro Americans. The two groups have mean sleep times of 6.05 hours and 6.85 hours. This finding has recently been discussed by Brian Resnick in National Journal and by our Steve Sailer.

Researchers reject a genetic explanation: "There is a consensus that innate biological differences between blacks and whites are not a factor" (Resnick, 2015). So what is the cause?

One study points the finger at racism:  "If you can take out that discrimination piece, the average African-American and the average Caucasian look at lot more similar. [...]  "It's not perfect, but in terms of sleep, a lot of the disparity goes away" (Resnick, 2015).

The study is by Tomfohr et al. (2012). It found that duration of deep sleep and duration of Stage 2 of light sleep correlated in African Americans with perceived discrimination, which is defined as "the extent to which an individual believes that members of his or her ethnic group have been discriminated against in society."

Nonetheless, as the authors note, sleep duration still differs significantly between African and Euro Americans even when the difference is adjusted for the effects of perceived discrimination. So we are left with a curious finding: two separate causes, one genetic and the other environmental, are producing the same pattern of effects. Both are reducing deep sleep and Stage 2 light sleep in African Americans while not affecting Stage 1 light sleep.

Whenever I see this kind of finding, I start looking for confounds. Is one cause a sock puppet for the other? It may be that perceived discrimination increases with African ancestry. Perhaps African Americans who feel conscious of discrimination also tend to be darker-skinned and more visibly African than those who don’t. This confound has actually been shown by several studies, such as the following:

This study tested the extent to which skin color is associated with differential exposure to discrimination for a sample of 300 Black adults. Results revealed that dark-skinned Blacks were 11 times more likely to experience frequent racial discrimination than their light-skinned counterparts; 67% of subjects reporting high discrimination were dark-skinned and only 8.5% were light-skinned. (Klonoff and Landrine, 2000; see also Keith and Herring, 1991)

Even if perceived discrimination could fully explain the race difference in sleep duration, we still couldn’t exclude a genetic explanation, since the degree of perceived discrimination is confounded with the degree of African ancestry.

In reality, perceived discrimination accounts for only part of the race difference, and since this difference remains significant even if we factor out that putative cause, the most parsimonious explanation is a genetic cause. Only that cause can fully account for shorter sleep duration in African Americans.

Studies in Africa

Another way to solve this puzzle is to look at Africans living in Africa. Do they show the same pattern we see in African Americans?

We know less about sleep patterns in Africa, but what we do know suggests that Africans, too, have shorter sleep duration. When Friborg et al. (2012) studied sleep in Ghanaians and Norwegians, they found that Ghanaians slept about an hour less than do Norwegians on weekends and between a quarter and half an hour less on weekdays. Oluwole (2010) studied sleep in Nigerian undergraduates and found they slept an average of 6.2 hours plus another 70 minutes in the afternoon. This pattern is actually typical in the tropical zone. People prefer to get some sleep when the temperature is at its peak and spend more time awake when it's more bearable.

But why would this pattern persist in African Americans? Perhaps it’s hardwired to some degree. When siestas become the cultural norm, there is selection for those individuals who enjoy being normal (and against those who don't).

Sleep patterns are heritable:

Assessed self-reported sleep data from 2,238 monozygotic (MZ) and 4,545 dizygotic (DZ) adult twin pairs born in Finland before 1958. Results indicate a significant hereditary effect on sleep length and on sleep quality. When the data were examined in subgroups defined by sex, age (18-24 yrs and 25+ yrs), and cohabitation status of the twin pair, the highest heritability estimates for sleep length were for Ss living together aged 25 yrs or older. For Ss living apart, the heritability estimates were statistically significant in all Ss aged 25 yrs or older. For sleep quality, significant heritability estimates were found for all groups except women living together. Results indicate that a significant proportion of the variance in sleep length and quality was due to factors that make MZ Ss more similar than DZ Ss. (Partinen et al., 1983)

A single genetic polymorphism seems to explain much of the variability between individuals in sleep patterns, particularly deep sleep and slow wave activity (SWA):

Here we show in humans that a genetic variant of adenosine deaminase, which is associated with the reduced metabolism of adenosine to inosine, specifically enhances deep sleep and SWA during sleep. In contrast, a distinct polymorphism of the adenosine A2A receptor gene, which was associated with interindividual differences in anxiety symptoms after caffeine intake in healthy volunteers, affects the electroencephalogram during sleep and wakefulness in a non-state-specific manner. Our findings indicate a direct role of adenosine in human sleep homeostasis. Moreover, our data suggest that genetic variability in the adenosinergic system contributes to the interindividual variability in brain electrical activity during sleep and wakefulness. (Retey et al., 2005)


So African Americans are getting enough sleep at night. It’s just that they're not getting enough afternoon naps. But aren't naps for kids? Or old fogeys? Actually, they’re quite normal for adults in much of the world. In the Nigerian study, 82% of the participants regularly took afternoon naps.

It’s ironic that the “r word” has been injected into this debate. If a behavior deviates from the white American norm, and if racism is held responsible either directly or indirectly, one is assuming that this deviation is pathological. It is “deviant.” It shouldn’t exist and something should be done about it. The white American norm thus becomes a norm for all humans, and all humans—if they want to be fully human—should strive toward it.

In reality, there is no single human nature. Genetic evolution didn’t slow down when humans began to split up and settle the different continents. It accelerated. And not just because our ancestors were adapting to different natural environments. Most of the acceleration took place long after the globe had been settled from the equator to the arctic. It happened when humans began to adapt to an increasingly diverse range of cultural environments. And those adaptations were mostly behavioral and psychological.

One of them is the way we sleep. The African sleep pattern is normal in its native environment. It is simply an adaptation to a particular set of circumstances, just as the northern European sleep pattern is an adaptation to another set of circumstances.


Friborg, O., B. Bjorvatn, B. Amponsah, and S. Pallesen. (2012), Associations between seasonal variations in day length (photoperiod), sleep timing, sleep quality and mood: a comparison between Ghana (5°) and Norway (69°). Journal of Sleep Research, 21: 176-184.

Keith, V. M., and Herring, C. (1991). Skin Tone and Stratification in the Black-Community. American Journal of Sociology, 97(3), 760-778.

Klonoff, E. A., and Landrine, H. (2000). Is skin color a marker for racial discrimination? Explaining the skin color-hypertension relationship. Journal of Behavioural Medicine, 23(4), 329-338.

Oluwole, O. S. A. (2010), Sleep habits in Nigerian undergraduates. Acta Neurologica Scandinavica, 121, 1-6.

Partinen, M., J. Kaprio, M. Koskenvuo, P. Putkonen, and H. Langinvainio (1983). Genetic and environmental determination of human sleep. Sleep: Journal of Sleep Research & Sleep Medicine, 6(3), 79-185.

Resnick, B. (2015). The Black-White sleep gap, National Journal, October 23

Retey, J.V., M. Adam, E. Honegger, R. Khatami, U.F.O. Luhmann, H.H. Jung, W. Berger, and H.P. Landolt. (2005). A functional genetic variation of adenosine deaminase affects the duration and intensity of deep sleep in humans, Proceedings of the National Academy of Sciences U.S.A., 102, 15676-15681

Sailer, S. (2015). Racism never sleeps: "The Black-White Sleep Gap: An Unexpected Challenge in the Quest for Racial Justice",  The Unz Review, October 29

Tomfohr, L., M.A. Pung, K.M. Edwards, and J.E. Dimsdale. (2012). Racial differences in sleep architecture: The role of ethnic discrimination, Biological Psychology, 89, 34-38.


Jon Claerbout said...

I wonder if sleep duration is correlated with IQ suggesting that if your brain is working hard you want more sleep. Just a wild guess.

Anonymous said...

Different species have different patterns of sleeping

Anonymous said...

Sorry, this one doesn't fly because Northern Europeans used to nap in the summers, too, and have biphasic sleep in the winters. They would wake at sunrise even if they'd only gotten four or five hours of sleep and make up part of the deficit after the big afternoon meal, between the heavy labor of the day and the light labor.