Access to Electric Light Is Associated with Shorter Sleep Duration in a Traditionally Hunter-Gatherer Community

Abstract

Access to electric light might have shifted the ancestral timing and duration of human sleep. To test this hypothesis, we studied two communities of the historically hunter-gatherer indigenous Toba/Qom in the Argentinean Chaco. These communities share the same ethnic and sociocultural background, but one has free access to electricity while the other relies exclusively on natural light. We fitted participants in each community with wrist activity data loggers to assess their sleep-wake cycles during one week in the summer and one week in the winter. During the summer, participants with access to electricity had a tendency to a shorter daily sleep bout (43 ± 21 min) than those living under natural light conditions. This difference was due to a later daily bedtime and sleep onset in the community with electricity, but a similar sleep offset and rise time in both communities. In the winter, participants without access to electricity slept longer (56 ± 17 min) than those with access to electricity, and this was also related to earlier bedtimes and sleep onsets than participants in the community with electricity. In both communities, daily sleep duration was longer during the winter than during the summer. Our field study supports the notion that access to inexpensive sources of artificial light and the ability to create artificially lit environments must have been key factors in reducing sleep in industrialized human societies.

Keywords: South American indigenous communities; artificial light-dark cycle; natural light-dark cycle; sleep timing.

Figure 1

Rest-activity profiles and sleep parameters differ between Toba communities living without access to electricity (NO-electricity) and with free access to electricity (Electricity). (A, B) Mean waveforms for the 2 communities during the summer (A) and winter (B). (C, D) Sleep parameters derived from sleep diaries combined with wrist actimeters for each community during the summer (C) and winter (D). *, †, and ¥: significantly different from NO-electricity (p = 0.05) for bedtime, sleep onset, and sleep duration, respectively; post hoc comparisons, Fisher's least significant difference test. Summer, n = 6 for both communities; winter, n = 8 and 11 for NO-electricity and Electricity, respectively. Dashed lines mark sunset and sunrise times. Data represent mean ± SEM.

Figure 2

Light-exposure profiles in Toba communities living without access to electricity (NO-electricity) and with free access to electricity (Electricity). Mean waveforms for the 2 communities during the summer and winter are shown. *Significant difference between groups (p < 0.05), post hoc comparisons, Fisher's least significant difference test. Data represent mean ± SEM.

Figure 3

Sleep parameters differ between winter and summer in Toba/Qom communities with and without access to electricity. Symbols as in Fig. 1C, D but significant differences are compared to the same community during the winter. Data represent mean ± SEM.