Most studies that have assessed the association between BMI and sleep have used self-reported usual sleep duration as their sleep measure. As noted previously, results from many of these studies have shown a simple inverse relationship between sleep duration and BMI, whereas results from others have shown a U-shaped relationship, with a higher BMI associated with both short and long sleep durations [21–23]. Other study results have shown increased morbidity and mortality risk among people reporting either short or long sleep durations [24–27]. We detected a strong association between days of insufficient sleep and BMI category except among people classified as underweight. A likely explanation for this exception to our overall finding is that the prevalence of conditions that both cause weight loss and disrupt sleep, such as eating disorders, cancer, or other chronic diseases, may be higher in the underweight group.
Some study results have shown that the association between obesity (or BMI) and sleep duration differs by sex; however, the direction of the difference has not been consistent. For example, Kripke et al. found a negative association between sleep duration and BMI among men but a U-shaped relationship among women ; Cournot et al. reported that short sleep duration was associated with higher BMI among women but not among men ; and results of a study conducted in Hong Kong showed a negative association between sleep duration and BMI among men but not among women . Longitudinal studies that have examined changes in weight with sleep duration or sleep problems have also been inconsistent. In a longitudinal study from Finland, sleep problems such as trouble falling asleep and trouble staying asleep were associated with major weight gain during a 5- to 7-year follow up among middle-aged women, but not men . In contrast, short sleep duration was associated with weight gain at 1-year follow up in a large Japanese study among men, but not women . Although we observed that women experienced more days of insufficient sleep than men, we saw a similar positive association between BMI and number of days of insufficient sleep among each.
We found that Hispanics had fewer days of insufficient sleep than blacks or whites. Results from the 2004-2007 National Health Interview Survey similarly showed that Mexican-Americans were more likely to experience long sleep duration than other race/ethnicities, although they also indicated that non-Hispanic blacks were more likely to experience both short and long sleep duration than were non-Hispanic whites . In the CARDIA Sleep Study, time in bed, sleep duration, sleep latency (time between going to bed and falling asleep), and sleep efficiency (percentage of time in bed spent sleeping) varied by both race and sex: white women had the longest sleep duration, highest sleep efficiency, and shortest sleep latency, and black men had the shortest sleep duration, lowest sleep efficiency, and longest sleep latency .
Individuals aged 65 years or older reported the fewest days of insufficient sleep in the present study, and individuals aged 25-34 reported the most. In contrast, results of an analysis of data from the 2004-2007 National Health Interview Survey showed that older adults were more likely to report short and long sleep durations and that younger age was associated only with long sleep duration . Among women enrolled as controls in the Collaborative Breast Cancer Study, age was negatively associated with hours of sleep ; the results of this study also showed that sleep duration was negatively associated with risk for obesity among women aged 50 or older as well as among those younger than 50 . We similarly observed a positive association between days of insufficient sleep and BMI category across all age groups.
The prevalence of frequent insufficient sleep was higher among current and former smokers than among never smokers and among those who reported no recent leisure-time physical activity than among those who reported some such activity. This association between unhealthy behaviors and insufficient sleep is supported by previous analysis of National Health Interview Survey data that demonstrated that smoking and physical inactivity were more prevalent among individuals who slept less than 6 hours compared to those who slept 7 or 8 hours .
Whereas most studies of the relationship between BMI and sleep have used sleep duration as the measure for sleep, we used days of perceived insufficient rest or sleep. Comparison of the measure used in our study with self-reported sleep duration, snoring, and daytime sleepiness in the same population will enable us to refine our conclusions. One limitation to using sleep duration as the sole measure for sleep is that such a measure does not address the quality of sleep. Even people who sleep for a relatively long time may not get adequate quality sleep because their sleep is disrupted by sleep-disordered breathing, sleep disorders such as insomnia, the side-effects of various medications, or other unknown causes. Results of studies comparing self-reported sleep duration with objectively measured sleep duration have shown that people with poorer sleep quality reported sleeping for shorter periods than those with better sleep quality, although the measured sleep duration for the two groups was the same [36, 37].
One recent study addressed the issue of sleep quality in an investigation of sleep duration and BMI. As part of the CARDIA Sleep Study, Lauderdale et al.  used wrist actigraphy to obtain objective measures of sleep duration and sleep fragmentation and also collected data on apnea symptoms, including snoring and tiredness. They found that both shorter sleep duration and greater sleep fragmentation were associated with higher BMI in unadjusted models, although adjustment for confounders (i.e., sociodemographic factors, smoking status, physical activity, and apnea risk factors) decreased the association between sleep duration and BMI and eliminated the association between sleep fragmentation and BMI, possibly because of the adjustment for snoring.
Our findings are subject to several limitations. First, the wording used in the insufficient sleep question is open to interpretation. For instance, respondents may interpret "enough" to mean at least a specific number of hours or rather sufficient time to awaken refreshed. The question also does not distinguish between "rest" and "sleep". Also, BRFSS data are collected through telephone surveys of the civilian, non-institutionalized population, therefore our findings are not generalizable to military personnel, institutionalized persons, and persons residing in households without landline telephones. Finally, the cross-sectional nature of the survey prevented us from attempting to determine the causal relationship between BMI and sleep. However, there is growing evidence that excess weight and insufficient and/or poor quality sleep may have a reciprocal causal relationship. Obesity has been shown to increase the risk for obstructive sleep apnea syndrome, a disorder characterized by frequent disruption of breathing during sleep caused by closure of the airways [38–40]. These abnormal breathing patterns result in disturbed sleep. Excess weight is strongly associated with the prevalence of sleep apnea, as well as with the frequency of disordered-breathing events and with oxygen desaturation [41, 42]. In recent years, results from a few prospective cohort studies have shown weight gain to be associated with an increased risk of developing sleep apnea [38–40], and clinical trial results have shown weight loss among sleep apnea patients to be associated with a decrease in the severity of sleep apnea [43–45]. On the other hand, there is also evidence that chronic sleep disruption may alter appetite regulation by changing levels of hormones such as leptin and ghrelin [17, 18]. One advantage to using BRFSS data, however, is that the large sample size of the BRFSS survey allowed us to assess the relationship between BMI and insufficient sleep for various subgroups based on sex, age, and race/ethnicity.