In 73 of the 519 nights included in analysis , nocturnal bouts of activity were detected by actimetry that were sufficient to divide nocturnal sleep into two bouts . In these cases, Actiware sofware chose sleep onset and wake times from the longer of the two sleep bouts to represent the sleep period for that night. If the following criteria were met, then the two sleep bouts were manually joined to allow reported sleep onset and wake time to represent one sleep period across the entire nocturnal period . Combining the two sleep periods did not change total nocturnal sleep duration, as any periods of awakening during the night were not included as sleep. Te combining procedure served to consolidate the sleep bouts into one sleep period with reduced sleep efficiency. Criteria for combining sleep periods were, the period of nocturnal activity must have occurred during a time when the subject was “usually” asleep/inactive , the subject must have been asleep for at least 2 h prior to the period of awakening, or the period of nocturnal awakening had to be shorter than the shortest period of sleep. For example, if a subject slept for 30minutes, awoke for 2h and went back to sleep for 6h, this would not qualify for fragmentation removal, and the 6h sleep period would be considered their nocturnal sleep period. However, if a subject slept for4h, awoke for 2h, and then slept for 4h, this would qualify for fragmentation removal .Dependent variables were calculated as follows. Actiware 6.0.9 sofware was used to score bedtime, sleep onset, wake time, rise time, nocturnal sleep duration , sleep efficiency , and nap duration. Nocturnal sleep duration and nap duration were summed to yield 24 h total sleep time . Activity data were imported into Clocklab 6.0 in 1min bins for calculation of non-parametric circadian variables, including L5 , M10 , relative amplitude , intradaily variability and interdaily stability . JMP 14 and Prism 7.0 were used for inferential statistics and to produce figures. Te primary aim was to assess the relationships between sets of independent variables and dependent variables using separate analyses of variance for each dependent variable.
Prior to conducting these ANOVAs, we explored the effects of potential covariates, such as age, number of co-sleepers,blueberry container size and body fat percentage by observing whether the potential covariates significantly correlated with the various dependent variables. Te only significant correlation observed was between age and sleep efficiency. Accordingly, to assess the relationships among community type and adult type with sleep timing variables and sleep duration variables, separate 2×3 ANOVAs were conducted for each dependent variable. To account for the significant relationship between age and sleep efficiency, a ANCOVA was used to emulate the ANOVAs, but with age entered into the model as a covariate. Tukey’s post hoc tests were used to further explore significant main effects of adult type, or significant interactions. In cases where parametric tests were not appropriate , Mann-Whitney U non-parametric independent samples tests were performed between community types, foregoing analysis by adult type. Statistical significance was defined at p<0.05 . Figures including means are plotted±standard error of the mean .Te Actiware 6.0.9 sleep scoring algorithm divided nocturnal sleep into multiple bouts on at least one night in 56% of the electric community sample and 47% of the non-electric sample, for a total of 73 out of 519 nights . Among individuals exhibiting sleep fragmentation, the average percentage of nights with fragmentation was 30% in the electric and 20% in the non-electric communities. Prevalence was highest in breastfeeding females in the electric communities. Fragmentation contributed to the nocturnal sleep efficiency score, which was 3.0% lower in the electric communities compared to the non-electric . An analysis of covariance model was used to test the effects of community type , adult type , and the covariate age on sleep efficiency. Te analysis showed a statistically significant interaction between the covariate age and community type =8.32, p=0.005). Sleep efficiency was related to age only in the non-electriccommunity. For this reason, separate tests were conducted for both electric and non-electric communities. In the electric community, there was a main effect of adult type =4.44, p=0.019), with lower sleep efficiency in males and breastfeeding females compared to females . In the non-electric community, there was no main effect of adult type =0.47 p=0.629).
Relationships between age and sleep efficiency by adult type were weak for both communities , and significant only for breastfeeding females in non-electric villages, with sleep efficiency decreasing with age =10.5, p=0.002). Subjective reports from interviews indicate that 90.7% of individuals in the electric and 92.3% in the non-electric communities, report waking up during the night. Te cause of sleep interruptions in both community types was most frequently attributed to infant care , and dogs barking . Despite this, the majority of individuals in each community reported feeling that they slept “enough”.Sleep timing, duration, and efficiency can be affected by environmental stimuli, including light, temperature, humidity, and co-sleepers. Residents in both communities went to bed approximately 3–3.5h afer sunset, which occurred between 17:17h−17:32h during the study, and awoke very close to sunrise, which occurred between 05:52h−06:00h . Wake times were on average closer to sunrise than to transitions in ambient temperature and relative humidity, which, at the time of recording, occurred after sunrise, as measured by iButtons in sleeping huts. A role for evening light exposure in the relationship between sleep onset time and sleep duration is suggested by a significant negative correlation between evening light and nocturnal sleep duration in the electric community . Evening light was not significantly correlated with sleep duration in the non-electric community , and morning light exposure was not significantly correlated with sleep duration in either group. In neither community was sleep efficiency or sleep duration significantly related to average nighttime temperature or humidity . Te absence of a relationship may be due to relatively low variability of average temperature and humidity across the nights of this study. A few of the sleeping huts in villages with electricity were constructed with tin and cement, and these had lower humidity at night compared to grass huts, but the small number of these huts precluded analyses by hut types.All participants in this study shared sleeping quarters with multiple children or adults. Higher numbers of co-sleepers might be expected to increase the number of nocturnal awakenings, and thereby reduce sleep efficiency and potentially sleep duration, as has been previously reported. In the present study, the average number of co-sleepers was slightly greater in the non-electric communities compared to the electric communities , yet the non-electric communities had both longer nocturnal sleep and higher sleep efficiency.
This is the first actigraphy study of sleep timing and duration in indigenous Melanesians living small scale, traditional horticultural lifestyles in the south pacific island nation of Vanuatu. We found that habitual sleep duration among the Ni-Vanuatu of Tanna Island is long compared to several small-scale hunter-gatherer, agrarian and pastoralist societies in Africa and Bolivia,raspberries in containers and compared to most samples from industrialized western populations studied by actimetry using Actiwatches . We also found that nocturnal sleep onset was delayed by 23minutes and duration was shorter by 28 minutes in participants living in coastal villages with on-demand access to electric light at night. A significant interaction with adult type suggests that the difference in sleep duration is driven primarily by breastfeeding females in the communities with electricity. Reduced nocturnal sleep in this group may have been causally related to increased light exposure during nighttime infant care, compared to breastfeeding females in the non-electric communities who had the same nocturnal responsibilities without on-demand availability of electric light. We interpret these results as supporting the view that sleep timing and duration in humans is shaped in part by lifestyle adaptations to the opportunities and challenges of particular ecologies. Hunter-gatherer and pastoralist societies, living at virtually the same latitude as Tanna Island and also studied using wrist-worn actigraphy averaged markedly less daily sleep than the Ni-Vanuatu on Tanna Island. Life on Tanna Island is characterized by reliable food access, a mild subtropical climate with relatively low daily and seasonal variability in temperature and day length, absence of predators, and minimal social conflict. Under these conditions, there may be no special fitness advantage of short sleep. Conceivably, there may be a fitness advantage favoring a short sleep genotype in hunter-gatherer and pastoralist societies that is not present in the horticulture-based lifestyle on Tanna Island. Alternatively, short sleep durations in some groups may reffect less favorable sleeping conditions, which could imply that these groups are chronically in sleep deficit. Te similar latitude, and thus daylength, sunrise, and sunset times, rule these out as explanations for differences between the Ni-Vanuatu and hunter-gatherer and pastoralist societies studied to date . We also interpret these results within the context of the developing economy sleep degradation hypothesis and the postindustrial sleep degradation hypothesis. Although sleep duration on Tanna Island was long by comparison with most actigraphy studies of industrialized western populations , sleep efficiency was low. This may reffect environmental disturbances, such as having multiple co-sleepers, and housing that offers little protection from surroundings . Compared to Western homes, the walls of dwellings in Vanuatu are thin and uninsulated and allow greater exposure to outdoor temperatures, and greater sound transmission when wild dogs bark or neighbouring babies cry. Reported sleep disruptions seem to reffect differences in location. For instance, the electric communities are closer to developed roads and reported more automobile related noise disruptions, which would be expected to increase as industrialization progresses. Noise is proposed as a large component of the ‘developing economy sleep degradation hypothesis’ since increasing population density paired with traditional housing offers little bufer. In addition, participants living in villages with electric lighting exhibited delayed and shorter nocturnal sleep. This was associated with increased exposure to evening light, and was particularly prevalent in breastfeeding females, who would be expected to experience more nocturnal waking and evening light exposure. Thus, sleep on Tanna Island exhibits characteristics of developing economy sleep degradation and post-industrial sleep degradation . While modern standards of living may improve sleep, access to lighting around the clock and other factors may counteract some of these improvements.Te difference in average nocturnal sleep duration between the electric and non-electric villages in our study sample was 28minutes. A reduction of this magnitude in industrialized populations is thought to be physiologically and behaviorally significant, especially if accumulating over days of the work week or longer. It is possible that the 7.88h average nocturnal sleep duration in Tanna Island villages without electricity represents a surfeit, and that a roughly half-hour reduction in villages with electricity is of no functional consequence. If 28min less sleep at night does represent a deficit, then we might expect to see an effort to compensate by increased daytime napping. While both study groups exhibited some daytime sleep, naps were significantly more prevalent in villages with electricity. When these naps were combined with nocturnal sleep to yield total daily sleep time, the difference in sleep duration between villages with and without electricity was no longer statistically significant. This suggests that daytime naps are at least in part compensatory and that the shorter nocturnal sleep duration in electric communities represents a deficit. Presumably, the magnitude of differences in sleep timing and duration between communities with and without electric lighting on Tanna Island is limited by continuous exposure to natural light throughout the day in both groups. Morning light opposes the phase delaying effect of evening light, and increased daytime light decreases sensitivity to artifcial evening light. Given the similarity of daytime light exposure patterns in the coastal and inland villages, evidence for a significant effect of on-demand electric light in coastal villages is notable. Another factor that may limit differences between groups is the use of solar torches after sunset in both communities. However, torches provide only low intensity light that is typically directed toward objects and away from the eyes. Despite the use of torches in the non-electric communities, those living in the electric communities showed more light exposure during the first hours of the night , and this was associated with delayed sleep onset times and less nocturnal sleep.