Our main objective was to test the SJH (positing that adolescents living in “traditional”, non-industrial environments will more closely fulfil their “biological/natural” sleep requirements25,26) by comparing sleep deprivation among adolescents in rural and urban societies. The SJH argues that adolescent “biological/natural” sleep quotas and circadian cycles can be ascertained from free days, when sleep patterns are minimally shaped by social commitments5,37. Therefore, we predicted that sleep deprivation would be rare in the more rural agricultural settings of Puebla and Campeche but more frequent among participants in Mexico City. Likewise, we predicted that we would not see sleep deprivation on free days among any of the rural participants.
Our predictions were not supported, instead, we found that short sleep quotas during school nights are common in both rural agricultural settings, with over 75% of adolescents in each group sleeping < 9 h/day. Furthermore, the proportion of short sleepers in Mexico City and Campeche (the latter being the most “traditional” setting in our study), are strikingly similar (94% vs. 98%). A substantial amount of agricultural Maya adolescents in Campeche also has short sleep quotas during free days (49%), compared to Totonac (26%) and urban Mestizo (29%) participants.
Disparities in school workloads, access to screen-based devices and sunset times might explain the predominance of short sleep quotas in Mexico City and Campeche on school days. Interview data show that, compared to adolescents from agricultural areas, urban participants have greater school workloads, so they are more likely to work late at night and go to sleep later. In addition, urban adolescents have ready access to screen-based devices, increasing their chances of staying awake longer. Analogously, solar data show that Maya participants in Campeche experience longer exposure to daylight due to later sunsets than in Mexico City and Puebla, which could help delay their sleep start.
Meanwhile, on free days, waking times are not determined by fixed schedules, gaining importance in the emergence of short sleep quotas. Adolescents who have late bedtimes (possibly due to social/recreational activities paired with the absence of parent-set bedtimes, as indicated by interview and ethnographic data) and whose wake-life activities prevent them from sleeping until late morning, are more likely to have short sleep quotas (such as strict training regimes, extra-curricular activities, farming/housework or religious life, as reported by participants themselves). Therefore, our findings in Campeche, where Maya participants have the highest proportion of short sleepers during free nights, suggest that adolescent sleep in more traditional, rural settings is embedded within social commitments and expectations rather than being free from them. Possibly, past and present adolescent sleep–wake cycles in other societies (i.e., horticulturalists, hunter-gatherers, pastoralists) have not been free from social expectations either.
It should be noted that sleep deprivation during school nights is significantly less acute in agricultural adolescents, who, according to actigraphy data, sleep 26–32 min longer than urban participants. In contrast, differences in sleep deprivation are minor during free nights, ranging from 4 to 17 min between sites. Disparities in short sleep quotas on school nights might arise from differing school schedules and commuting times, as Totonac and Maya participants start classes 30 min later than urban teenagers in Mexico City and have shorter commutes.
Variability in sleep deprivation could partially explain why the proportion of participants reporting recurrent daytime sleepiness is lower among Totonac and Maya adolescents. However, the perception of drowsiness could also vary depending on the tasks socially sought and reproduced at each site. In this sense, a study by Eide and Showalter38 estimating the “optimal” sleep period to maximize reading and mathematics test performance in U.S.A. students aged 10–19 found that “optimal” sleep quotas changed with age and were dependent on the level of complexity of the task. Therefore, urban participants, who might be under more pressure to optimize academic performance, as indicated by interviews and ethnographic data, might require more sleep to maintain attention on complex cognitive tasks for longer. Furthermore, subjective feelings of sleepiness may be stronger in urban adolescents who perceive academic activities as dull or repetitive15. Still, these hypotheses remain speculative and further studies are required to test them. Additionally, in agreement with the Sleep Continuity Theory39 (which states that a minimum of 10 min of interrupted sleep is needed to serve a restorative function), variation in self-rated drowsiness could reflect differences in participants’ sleep efficiency (i.e., the ratio between the total time spent asleep and the total time in bed). While this matter is beyond the scope of this paper, future research will examine whether sleep efficiency is higher in both indigenous rural settings than in Mexico City.
Considering that daily sleep quotas are normally distributed in human populations, using cut off values to assess sleep sufficiency is problematic because it assumes optimal sleep is monolithic rather than variable15,40. Thus, we evaluated sleep curtailment during school days as a complementary approach to check sleep sufficiency5,16. Again, we found that sleep curtailment is pervasive in rural, agricultural adolescents, although significantly less than in Mexico City, where values are between 17 and 25 min higher than in Puebla and Campeche. As with short sleep, differences in curtailment values are likely due to specific school workloads, access to screen-based devices, sunset times, earlier school schedules, longer travel times to school and adolescents’ likeliness to sleep in during free days. Altogether, our findings undermine the premises of the SJH since they suggest that, given certain socio-cultural and ecological factors, adolescents from rural, agricultural societies may also express short sleep quotas.
When we examined which specific bio-socio-cultural factors influenced nightly sleep duration in the three study sites, we found that advanced puberty had a negative impact on sleep duration during school nights, but not on free nights. This effect is most probably related to personhood associated with age rather than to sexual maturation as older teenagers usually build social bonds outside the household, gain social responsibilities (such as household or farming work) and acquire the freedom to set their own bedtimes, which in turn results in shortened sleep during school nights4,41,42. Arguably, such changes are decidedly more marked in subsistence rural than postindustrial urban teenagers, as indicated by interview, ethnographic and bibliographic data43,44. Maya and Totonac adolescents in Campeche and Puebla allocated more time than children to farming and domestic tasks to contribute to their nuclear family economy and parents. Notable, Kramer (2002) has reported that Maya girls from the same site in Campeche, increase their level of production in early adolescence (generally by age 12–13) while boys increase their work efforts later in youth (generally by age 16–17)44, resembling pubertal maturation. Similar age and sex differences in work efforts might also prevail in Puebla. In comparison, teenagers in Mexico City reported spending their time either learning or in leisure activities, not regarding their age, all of which could explain why the effect of advanced puberty was significant in Campeche and Puebla but not in Mexico City. Results from previous meta-analyses agree with ours inasmuch they note that sleep duration decreases with age on school days but remains the same from pre-puberty to late adolescence on free days5,16.
Along with age, personhood is molded by gender, a cultural variable that previous research has found to have confounding effects on adolescents’ sleep duration45. Given that the participants of this study belong to three distinct cultural groups, we expected to encounter mixed effects of gender on sleep in our samples. Surprisingly, gender consistently influences sleep duration across all study sites and weekdays, with girls sleeping more than boys. Differences in sleep quotas could be linked to distinct gender access to screen-based devices, involvement in household activities, bedtimes routines, and time investment in getting ready for school. Notably, gender effects are larger on school days, when waking times are fixed, and also in agricultural environments, where greater social distinctions between girls and boys might exist. In this sense, our results concur with previous studies indicating that human sleep patterns are heavily shaped by the interaction of gender and socio-economic status46,47. Still, more research is required to clarify whether these findings could also be explained by sexual dimorphism, as some other studies have suggested48.
A third factor affecting participants’ sleep was chronotype, a behavioral trait that, unlike gender and sex, has been extensively investigated in relation to adolescent sleep patterns. Specifically, researchers have described a chronotype transition towards eveningness around puberty49 that is considered a marker of adolescent sleep–wake cycles6 which is itself then conducive towards sleep deprivation on school days17. However, we did not find a significant effect of evening chronotype on sleep quotas during school nights, but instead, found a significant positive effect during free nights. The cumulative impact of nocturnal sleep curtailment could partially explain why evening types sleep longer on free nights, presumably to recover, even though their sleep is not pointedly reduced on school nights10. Chronotype has the most marked effect on sleep duration in Mexico City, but not in the rural locations, where evening chronotypes are less frequent and adolescents are less prone to sleep in during free days. Similar differences in chronotype between urban and rural populations have been previously described in adolescents and adults30,50,51. These findings suggest that differences in social activities affect human chronotype expression and variability.
Daytime napping is another behavioral trait known for shaping nightly sleep quotas52. The propensity of contemporary humans to experience a mid-afternoon dip in alertness (linked to circadian oscillations of body temperature and performance) is well recognized33,53. Since it has been hypothesized that a napping episode during the mid-afternoon would occur under “natural”, rural conditions but would be inhibited by industrial lifestyles22,33, we expected to find higher napping rates in agricultural adolescents than in urban participants. Additionally, we reasoned we would observe greater nap ratios during school nights, derived from nightly short sleep quotas54. None of these predictions was supported by our results. Napping behavior is infrequent in all our study sites and we found no significant differences between weekdays (non-paired t, p = 0.95). This suggests, napping episodes are opportunistic and do not necessarily reflect participants alleged “sleep debt”. Even so, compared to teenagers in Campeche and Mexico City, participants in Puebla, who sleep the most during the night, have the lowest napping rates (probably because they sleep the most during the night). This distinction might be why, although napping behavior consistently shortened sleep duration across weekdays, its impact was no significant in Puebla.
Along with individual characteristics and behavioral preferences, the physical and social features of sleep environments, such as temperature, lightening and co-sleeping practices, also act as sleep modifiers55. For instance, thermal stress, caused by exposure to extreme temperatures, results in difficulty falling asleep and frequent awakenings56. Our results confirm this, given that higher temperatures in hot weather or in isolated, temperate sleeping environments (Campeche and Mexico City) give rise to shorter sleep durations but facilitate longer sleep in less shielded, colder sleeping environments (Puebla). Similarly, the effects of light on nightly sleep have been profusely addressed in the relevant literature4,6,17,25,57. In this regard, we presumed that daylight duration and intensity (estimated through clear sky conditions) would predict adolescent sleep duration in rural settings, but not in Mexico City, where buildings are designed for buffering external environmental cues. We confirmed that, as day length increases, participants’ sleep decreases in Puebla, but we did not find this effect in Campeche. This distinction is most probably related to seasonal differences in agricultural activities. At the time of the study, Totonac participants in Puebla had more work to do during daylight hours (i.e., maintaining, harvesting, and cleaning their crops) than Maya participants in Campeche who were engaged in weeding their lands and waiting for the harvest. We also observed that as daylight intensity increased, participants’ sleep would decrease in all sites. Daylight at high intensities is recognized for advancing or delaying sleep depending on whether the individual was exposed to morning or evening/night light, respectively57. Although we cannot infer the exact time of participants’ exposure to daylight, we can assume that on non-cloudy, rain-free days, adolescents could perform diverse outdoor social and physical activities which would have inhibited sleep propensity and negatively affected sleep duration.
We hypothesized that the exposure to artificial lights at night would shorten sleep during school days, with a marked effect across sites given previous studies have reported light exposure is associated with shorter sleep durations17,23,58,59. Surprisingly, exposure to light at night has mixed effects on sleep quotas depending on its intensity and social context. For example, while standard domestic ambient light at night (< 500 Lux) increases sleep duration on school nights, dim ambient light at night (< 20 Lux) shortens sleep quotas in Mexico City and increases them in Puebla. Moreover, nocturnal illumination does not affect sleep in Campeche, potentially because of participants’ tendency to sleep in dark environments and their limited access to screen-based devices, mobile/cell phone coverage and the Internet.
Bringing adolescent sleep into its socio-cultural context together with its emotional dynamics is essential for understanding our contrasting results. Urban and rural participants would commonly report falling asleep under lightened conditions, either because they were watching images on their TV/phone/tablet or because those with whom they shared rooms would keep the lights on until late at night. In particular, agricultural Totonac participants in Puebla emphasized that having a light on while sleeping made them feel safe at night, a dangerous period when natural and supernatural characters (e.g., human and non-human predators, sorcerers, evil spirits or dead people) lurk in the shadows. Hence, adolescents whose sleep was exposed to < 500 Lux on school nights probably began sleeping before domestic lights went off due to exhaustion (i.e., strong sleep pressure) and the lights’ inhibitory effect on arousing emotions, two factors that would promote sleep. On the other hand, lights < 20 Lux mainly coming from electronic media or undesired luminous pollution in Mexico City would presumably elicit arousal among urban participants, thus shortening their sleep. In contrast, exposure to < 20 Lux resulting from moonlight, streetlights and domestic lights in Puebla would have helped to inhibit arousal among Totonac adolescents, facilitating longer sleep quotas.
Assisted awakening and social sleep practices provide further evidence of the interrelationship of adolescent sleep and its psycho-socio-cultural context. Even though assisted awakening tends to shorten sleep duration on school nights, an opposite trend is observed in Campeche. It is possible that, as Maya participants are generally awakened by the sun, dogs barking, roosters or noise of other household members, they would only have needed assistance to wake up when all other awakening cues had failed. This would mean that, contrary to what happened in Mexico City or Puebla, participants who need assistance for awakening in Campeche end up waking later and, thus, sleeping longer. Lastly, social sleep, commonly practiced in agricultural settings, has a marked positive effect on sleep duration in Puebla, while tending to shorten sleep in Mexico City and Campeche. As above, the participants’ cognitive-emotional state might underlie the distinct effects of social sleep29. Unlike urban and Maya participants, adolescents in Puebla would have found sleeping with others comforting and reassuring, inhibiting alertness and enabling prolonged sleep quotas.
Taken together, our results suggest that the prevalence of adolescent short sleep quotas is not less in rural, agricultural and more traditional environments than in post-industrial urban societies. These findings bring into question current assumptions about sufficient sleep and how adolescents slept before the modern era. Contrary to the SJH, reduced adolescent sleep durations might have been a constant in our species’ evolutionary history where individuals weighed the costs of reduced sleep against the benefits to them or their group of economic, social, reproductive or rearing waking-life activities22,60. Consequently, we advocate further research to delve into the relationship between sleep and health outcomes in non-clinic, rural settings to better understand sleep’s role in our evolutionary history.
Additionally, our study highlights the influence of ontogenetic development on the expression of human chronotypes, where a combination of genetic and epigenetic factors (potentially modulated by bio-socio-cultural factors such as photoperiod, temperature, developmental stressors, lifestyle, or parental involvement in offspring sleep) give rise to distinct circadian rhythms from prenatal development to old age61. Notably, despite the role of circadian rhythms in the maintenance of cognition, behavior and mental wellbeing, the study of epigenetic mechanisms is recent and mainly focused on non-human models62. Thus, we stress the need to incorporate a developmental approach to the study of infant and adolescent sleep to shed light on the epigenetic regulation of human biological rhythms and its short- and long-term consequences in health.
This research is subject to limitations. Firstly, our sample size was small, limited by convenience sampling in rural and urban settings. Secondly, we lacked longitudinal data, critical for drawing comparisons between different age groups and identifying sleep developmental trajectories. Additionally, we could not incorporate data on seasonal variations in diet, energy expenditure and allocation, which might impact sleep traits. Thirdly, the Morningness-Eveningness Reduced Scale (MERS), employed for assessing the phase preferences of participants over a 24-h day, requires individuals to structure time in hours, minutes, and seconds, and not as a function of socio-ecological cues (such as sunrise, sunset, meal times, non-human animal behavior, radio or TV programming, etc.). Although most of the study participants were familiar with “modern” uses of time, some were not, which might have reduced MERS accuracy. Similarly, the PDS, a self-reported instrument for evaluating pubertal status, may fail to reflect the precise developmental stage of adolescents compared to Tanner stages.
This study provides novel evidence about variation in adolescent sleep quotas through the examination of sleep in one post-industrial, densely urban and two agricultural indigenous sites in Mexico. We advocate further sleep research employing an ecological, cross-cultural perspective to broaden our understanding of human sleep development, variability, health and evolution. Such an approach could help guide future research agendas that translate into more equitable and effective health policies and practices for child and adolescent wellbeing.
Source: Ecology - nature.com
