Magazine Response to de la Iglesia

We wish to respond to the commentary of de la Iglesia et al. [1]. Studies comparing sleep in different communities have different goals. One frequent goal has been to determine how sleep is affected by manipulating specific ‘modern’ conditions. Many studies have investigated the effect of artificial light and electronic entertainment. Such studies have clearly shown that light, particularly blue light, delays sleep onset [2]. Studying the effect of artificial light on sleep was not a goal of our study. Our goal was to study the nature and timing of sleep of pre-industrial humans [3]. Since humans migrated out of Africa, the environment to which they are exposed has changed radically. In particular, during the last century not only artifi cial light, but also sleeping surfaces, central air conditioning and heating, soundproofi ng, predation risk, insect exposure and innumerable other aspects of the sleep and waking environment have been transformed. Some of these alterations may have increased sleep, whereas others may have decreased sleep. These changes are far too rapid for us to have evolved adaptations to them. The data we collected and presented in considerable detail can readily be compared to data gathered with the same instruments and processed with the same algorithm in modern societies (see Supplemental Information in [3]). Environmental changes may contribute to sleep pathologies including insomnia, which affl ict up to 20% of the post-industrial population [4,5]. One cannot fully understand these pathologies without knowing the evolutionary history of sleep, much as a good clinician seeks to determine the development of a sleep disorder in a patient. Correspondence The cited study by de la Iglesia et al. [6] in the Toba/Qom is diffi cult to compare with our study. We restricted our sample to subjects aged 20–55 years (average was 37). This is an age range with relatively stable sleep parameters, facilitating comparison with other studies. In the Toba/ Qom study [6], by comparison, the average ages of two of the four groups listed were 17 and 27, with some subjects as young as 14. It is well known that sleep duration is much greater in teenagers than in adults, so one would expect much higher average sleep durations in that study than in ours, even if the adult population means were identical. Further, their Toba/Qom subjects were described in the following way: “Most adults are unemployed and subsist on governmental subsidies and/ or very short-term temporary jobs.” There is evidence that unemployment in the United States is associated with increased sleep [7]. They do not present health or BMI (bodymass index) data on their subjects or environmental temperature data. But a paper published in the same year by this group describes the Toba/Qom as “a population in transition,” with a 45% obesity rate [8], comparable to that of the most severely affected populations in the United States (http:// www.cdc.gov/), along with very high levels of hypertension. The prevalence of metabolic syndrome (greatly elevated risk for, and incidence of, heart disease, diabetes, and stroke) in the Toba/Qom is 38% [8]. We feel that our study populations are much more informative for extrapolating to those of ancestral humans because of the heavy reliance on foraged and handcultivated food sources in all three of our population groups, and the minimal infl uence of government subsidies. Table S1 in Supplemental Information published with this article online displays the average sleep duration by population and season of the studies referenced in our paper and in the de la Iglesia et al. commentary [1], all using the same methodology. The study of rubber tappers mentioned in the critique adds to a vast literature on sleep among different employment groups and ethnicities, but does not speak to the validity of our observations. It also does not present actigraphic data on sleep