Sleep variability and nighttime activity among Tsimane forager-horticulturalists

Abstract

Objectives: A common presumption in sleep research is that "normal" human sleep should show high night-to-night consistency. Yet, intra-individual sleep variation in small-scale subsistence societies has never been studied to test this idea. In this study, we assessed the degree of nightly variation in sleep patterns among Tsimane forager-horticulturalists in Bolivia, and explored possible drivers of the intra-individual variability.

Methods: We actigraphically recorded sleep among 120 Tsimane adults (67 female), aged 18-91, for an average of 4.9 nights per person using the Actigraph GT3X and Philips Respironics Actiwatch 2. We assessed intra-individual variation using intra-class correlations and average deviation from each individual's average sleep duration, onset, and offset times ( ɛ¯).

Results: Only 31% of total variation in sleep duration was due to differences among different individuals, with the remaining 69% due to nightly differences within the same individuals. We found no statistically significant differences in Tsimane sleep duration by day-of-the-week. Nightly variation in sleep duration was driven by highly variable sleep onset, especially for men. Nighttime activities associated with later sleep onset included hunting, fishing, housework, and watching TV.

Conclusions: In contrast to nightly sleep variation in the United States being driven primarily by "sleeping-in" on weekends, Tsimane sleep variation, while comparable to that observed in the United States, was driven by changing "bedtimes," independent of day-of-the-week. We propose that this variation may reflect adaptive responses to changing opportunity costs to sleep/nighttime activity.

Keywords: Tsimane; actigraphy; opportunity costs; sleep; traditional society.

Figure 1

Variation in sleep onset is shown as a function of the number of nights of sleep recorded for each participant (best fit line y = 25.2 + 4.7*x). The profile of sleep recording lengths is bi-modally distributed around 3 nights and 7 nights, which is insufficient to test for a non-linear relationship. However, since a significant linear relationship was found, we include “number of nights recorded” as a control variable.

Figure 2

Sleep duration (in minutes) by day of the week. Boxplots show 25^th percentile, median (black line), and 75^th percentile. Day of the week refers to the day the person woke up, not the day they went to bed (i.e. Saturday night through Friday night).

Figure 3

The intra-personal variation in sleep duration, onset, and offset for men and women. Men’s values are plotted on the top row, in blue, and women’s on the bottom, in red. 15 minute bins were used for all three plots.

Figure 4

Night-to-night variation in sleep duration over the adult life course for men (blue triangles) and women (red circles). Each data point reflects an individual’s average deviation from their personal average sleep duration ( ${\overline{\mathrm{\epsilon }}}_{\mathrm{i}}$).

Figure 5

Sleep onset (purple circles) and sleep offset times (orange triangles), for women (top) and men (bottom). Each horizontal entry represents a different individual, vertically sorted by descending nightly sleep duration. 95% prediction intervals, calculated using individual average sleep onset and sleep offset times, are shown with solid lines.