Physical activity is a complex behavior which plays an important role in diabetes prevention and can be influenced by a number of biological, psychosocial, and environmental factors. We tested the hypothesis that free-living urban adults with parental history of type 2 diabetes who habitually sleep short hours will have reduced daily physical activity. Indeed, multiple linear regression analysis of the data from our entire study sample showed a significant positive association between objectively-measured habitual sleep duration and the total amount of daily body movement (). In addition, compared to a similar group of participants with average sleep duration ≥6 h/d (), those who slept <6 h/d had significantly lower amounts of total body movement, spent less time engaged in moderate-plus-vigorous physical activity, and were more sedentary despite having more daily waking time available to them ( and ). These findings are in agreement with the hypothesis that sleeping short hours could be associated with reduced everyday physical activity, which may contribute in part to the relationship of chronic sleep insufficiency with increased incidence of obesity and diabetes (5
Based on wrist actigraphy data, nearly half of the participants at the Chicago site of the CARDIA study had habitual sleep duration <6 h/day (4
). Consistent with these observations, 40% of the consecutively enrolled young adults with parental history of type 2 diabetes in our convenience sample had objectively-measured short sleep (). Evaluation of these short sleepers by laboratory polysomnography revealed that they were able to sleep as much as the subjects in the reference group during one night of inpatient monitoring and did not suffer from any sleep disorders that could diminish the quantity and quality of their sleep (). Instead, short sleepers had significantly higher subjective daytime sleepiness and a tendency towards more consolidated sleep with fewer arousals during overnight polysomnography consistent with the presence of a behavioral pattern of habitual sleep curtailment and chronic sleep debt.
So far, large cross-sectional analyses of the relationship between habitual sleep duration and physical activity in adults have used self-reported data which have limited reliability (8
) and can lead to conflicting results showing either positive (10
), negative (13
), or no significant association (14
). Few studies have examined the effects of experimental sleep deprivation on objectively-measured physical activity. Schmid et al. studied healthy young men who were exposed to one night of experimental sleep restriction and found a decline in movement counts with a shift from higher intensity towards lower intensity daytime activities (25
). Brondel et al. imposed overnight sleep restriction followed by structured exposure to physical activity and laboratory meals during the first half of the day and free-living activity in the afternoon and evening, and reported opposite results (26
). Finally, Roehrs et al. found that overnight sleep deprivation resulted in a higher percentage of inactivity time during the next day (27
), whereas Bosy-Westphal et al. did not detect effects of sleep restriction on daytime activity assessed by heart rate monitoring and pedometry (28
). Unfortunately, all of these experiments involved acute sleep restriction, which may have different effects when compared to the continuous impact of chronic sleep curtailment on human physical activity. Nevertheless, the reduction in objectively-measured activity of free-living adults who curtail their sleep in our study () resembles the effects of experimental sleep restriction described by Roehrs et al. (27
) and Schmid et al. (25
). While this similarity raises the possibility that chronic sleep curtailment may result in reduced amount and intensity of everyday physical activity, the observational nature of our data does not allow such inference. Indeed, causality could also flow in opposite direction whereby adoption of a sedentary lifestyle (e.g. one involving increased TV watching or use of other information, entertainment, and social media outlets) could lead to secondary sacrifice of sleep and physical activity (29
Irrespective of the underlying causal mechanisms, habitual sleep curtailment in our group of young urban adults at risk for developing type 2 diabetes was associated with 27% lower levels of total body movement and nearly 45 min less daily moderate-plus-vigorous physical activity ( and ). If confirmed by larger population-based studies, these findings could have important public health implications. Higher amounts of total body movement have been associated with improved insulin sensitivity, better glucose tolerance, and reduced incidence of type 2 diabetes (17
). In addition, moderate and vigorous-intensity physical activity has a well-established role in the prevention of type 2 diabetes (3
). For example, walking for exercise at least 2.5 h vs. less than 1 h per week in the Finnish Diabetes Prevention Study was associated with a more than 60% lower risk of incident diabetes (31
). Furthermore, current guidelines recommend at least 30 minutes of moderate-intensity physical activity at least 5 days a week or 30 minutes of vigorous-intensity physical activity at least 3 times a week in order to reduce cardiometabolic risk in susceptible individuals (32
). Thus, future efforts to define the mechanisms which contribute to the association of chronic sleep insufficiency with chronic metabolic morbidity should include objective measurements of the amount and intensity distribution of habitual physical activity.
The amount of daily sedentary time, itself, has been associated with insulin resistance, abnormal glucose metabolism, and increased metabolic risk (19
). Our short sleepers had longer waking hours at their disposal, but devoted the entire waking time gained by sleep curtailment to various sedentary behaviors. When we examined the total amount of 24-hour sedentary time (including the amount of daily sleep which was longer in the reference group), short sleepers still spent more than 1 extra hour each day engaged in sedentary behaviors (). Data from the American Time Use Survey suggest that short sleepers may trade sleep in exchange for commuting, work, socializing, watching television, or using other media (33
). The types of sedentary behaviors that are favored by young urban adults at risk for type 2 diabetes who curtail their sleep remain to be explored. Additional experimental work is also needed to determine whether chronic sleep insufficiency may increase the propensity of individuals with pre-existing risk for obesity and type 2 diabetes to adopt increasingly sedentary lifestyles.
Our study has several strengths and limitations. We collected proof-of-concept data from a carefully screened sample of healthy individuals at risk for type 2 diabetes, while avoiding the confounding effects of obesity and poor general or emotional health. The use of laboratory polysomnography and objective monitoring of habitual sleep and free-living physical activity also allowed us to exclude the presence of sleep pathology and avoid assessments based on imprecise self-reports of these behaviors (see Results for an example of systemic bias in self-reported sleep duration). Finally, it was important to study a population with high risk for type 2 diabetes in order to inform future behavioral and primary prevention research focused on sleep and metabolic risk reduction. Despite its strengths, this study included a relatively small number of participants whose free-living behavior was monitored during a single period of time. Although our objective sleep data were similar to those of population-based reports (4
), study participants were not randomly selected and results may not be entirely representative of the relationship between sleep and physical activity in this high-risk group. Finally, while wrist actigraphy offers a valuable non-invasive approach for monitoring the habitual pattern of nighttime rest in free-living individuals, it does not provide direct measures of polygraphically documented sleep. Nevertheless, this methodology is well-validated in normal sleepers and provides surrogate estimates of nighttime sleep that correlate strongly with corresponding polysomnography-based assessments (20
In conclusion, our findings suggest that a large number of healthy young adults with increased risk for type 2 diabetes habitually curtail their sleep. Compared to participants with ≥6 h of daily sleep, those who slept less were more sedentary and had reduced amounts of total body movement and time spent in moderate-and-vigorous physical activity. Larger studies are warranted to explore the hypothesis that reduced physical activity may contribute to the association of chronic sleep insufficiency with incident obesity and type 2 diabetes.