- Introduction
The prevalence of obesity has been on a steady increase in the last 3 decades to the extent that it has become a major public health concern (Bell & Zimmerman, 2010). This trend has been attributed to environmental factors that prevent or inhibit healthy eating and active life. Parallel to the prevalence of obesity and overweight has been the reduction in sleep duration. The longer working hours, in a bid, to make more money, and the growing 24-hour internet and cable TV entertainment have led to many people sleeping fewer hours than the recommended 6-8 hours . This has led to the emerging and growing evidence to suggest that chronic sleep restriction/reduction contributes to obesity. This has informed some novel intervention strategies that specifically target short sleep duration. Unfortunately, the actual mechanism linking sleep restriction and obesity is not clearly understood . This paper shall briefly present evidence from recent research focusing on reduced sleep as an obesity risk factor in both children and adults. The paper will also outline the proposed mechanism of sleep as an obesity risk factor. Before delving into the correlation between sleep and obesity, it is necessary to define obesity and review the related statistics.
2.0 Definition of Obesity and the related statistics
Obesity is defined as having a sex- and age- specific body mass index (BMI) at or above the ninety-fifth percentile of national growth standards. Obesity is directly linked to excessive/abnormal fat accumulation. The BMI is a ratio of the body weight and height. It is the most common and accurate measure of obesity because it correlates to the amount of body fat. In terms of BMI, a person is considered to be obese if s/he has a BMI of over 30 (which is often ≥20% the normal body weight) and overweight if s/he has a BMI of between 25 and 29.9. A person with a BMI of 40 and above is said to have morbid obesity (Bell & Zimmerman, 2010). Obesity is associated with chronic illnesses such as diabetes, cardiovascular disorders, hypertension, gallstones and cancer .
The dramatic increase in the prevalence of obesity between the 1980s and 1990s was a wake-up call to the world to take deliberate steps to mitigate the situation. In 2008, 35% adults aged 20 years and above were overweight, and 11% were obese. Compared the 1980s, the world obesity prevalence among children and adolescent has doubled. Up to 17% of children and adolescent were obese in 2003/2004 while 34% were overweight. In 2011, more than 40 million children under the age of 5 years old were overweight.
These grim statistics has been the driving force behind the plethora of research on obesity, all in a bid to identify modifiable risk factors that can be targeted in the design of effective interventions. Of interest to this paper is reduced sleep as an obesity risk factor, which has been investigated and well documented in the last couple of years.
- Studies linking reduced sleep to obesity
There is a growing body of evidence and a myriad of studies spanning five continents that have established that reduced sleep is a risk factor that could be targeted in the prevention of obesity . Over the past fifty years, probably due to busy lifestyles, severe reduction in sleep has paralleled the increase in the prevalence of obesity and other chronic illnesses in the western nations. Adequate sleep (AS) has been perennially linked to the health, health-related behaviors and well-being of children and adolescents .
A cross-sectional study was conducted in Tao-Yuan County, Taiwan, to investigate the correlation between adequate sleep (AS, regular 6-8 hours sleep per night) and frequency of doctor’s visit, excessive body weight and health-related behaviors among adolescent. The study, involving 656 participants aged between 13 and 18 years (53.2% boys and 46.8% girls) established that 54% of the subjects slept less than 6-8 hours. The participants with reduced sleep had a lower appreciation of life, were not effective in stress management, did not exercise regularly, did not adopt a healthy diet and did not take responsibility for their health seriously. The study also found that higher frequency AS correlated to lower frequencies of obesity. Thus reduced sleep duration among the adolescent is not only a risk factor for obesity but for other unhealthy behaviors as well, which also contribute to obesity and other diseases . The limitation of this study was that it was cross-sectional descriptive design and thus not very reliable in making conclusion on causality. However, the strength of the study was in the fact that it investigated several other health indicators and behaviors which may be associated with obesity and other conditions.
Another study by Bell and Zimmerman (2010) sought to evaluate the correlation between night time and daytime sleep duration and obesity in children and adolescent. The researchers used existing national, longitudinal, panel survey data collected for the two groups to evaluate if poor sleep in early years of life had lasting effects on obesity 5 years down the line. They also investigated if there was a difference in the association between sleep and obesity among preadolescents and adolescents and if daytime and nighttime sleep independently affect subsequent obesity. The study was carried out between 1997 and 2002 and involved subjects aged between 0 and 13 divided into two cohorts, younger children (0- 4 years old at the start of the study-822 children) and older children (5-13 years old-983 children). While short period of nighttime sleep at the baseline was found to increase the risk of subsequent obesity among the younger group, it was not the same for the older group. In addition, daytime sleep duration had an insignificant effect on subsequent obesity and thus napping is no substitute for nighttime sleep. The study established that inadequate sleep at the infancy and pre-shool age is a lasting risk factor for subsequent obesity (obesity in later years). The strength of the study was that it employed the longitudinal study design which is more credible in drawing conclusions on causality. In addition, the data source was from a large nationally representative sample. However, the source of the data (panel survey income dynamics child development supplement-PSID CDS) introduces limitations in interpretation of the results because the PSID CDS only collects sleep data for 2 days in a year. As such, the reported sleep patterns are not accurate representative of the usual sleep patterns for the entire year
4.0 Adequate sleep and obesity in children
Scholars have gone further to establish that infants who sleep less than 12 hours are at risk of getting obese as early as at the age of 3 years. Gillman, Rifas-Shiman, Kleinman, Oken, Rich-Edwards, & Taveras (2008) studied how four modifiable pre- and postnatal factors influence childhood overweight. The four risk factors evaluated in this study were gestational weight gain, maternal smoking during pregnancy, infant sleep duration and breastfeeding duration. The study involved 1,110 mothers and children and studied 16 combinations of the above mentioned risk factors. It concluded healthy levels of the four factors during the early stages of life lowered the probability of overweight at the age of 3 years. These findings agreed with those of another, involving 950 children that established that night sleep of less than 12 hours during the early infancy is a risk factor for overweight and adiposity at age 3 (pre-school period). The strength of these two studies was that they used prospective data collection which allows for adjustment of several related confounding factors. The main limitation of the two was in the outcome; overweight at age 3, at which overweight does not predict overweight later in childhood or adult consequences. A similar but larger study in the UK, following 8,234 children for 7 years from birth, showed that those who slept for less than 10.5 hours at night at the age of 3 years had a 45% higher probability of getting obese by age 7 unlike their counterparts who slept more than 12 hours a night. Unlike the two earlier studies, the UK study had a larger sample size and followed the participant for longer (7 years), making the data more reliable in making the conclusion. The limitation of the study is that it is a cohort study, this kind of studies can not identify causality but association. Another limitation was the fact that the study was aimed at making comprehensive conclusion for the UK population, but the minority groups were slightly under-represented.
Reduced sleep at childhood not only has an impact on the child’s weight during childhood but may have a prolonged effect on weight into adulthood. In New Zealand, researchers followed 1,037 children to the age of 32 from birth. They collected data on the average sleep duration from parents at ages of 5,7,9 and 11 and found that every hour reduction in sleep during childhood was linked to a 50% higher risk of obesity at the age of 32 years. The strength of this research was that it followed the participant for a very long time (32 years) which makes the data more reliable. However, the data on average sleep was not representative of the entire childhood sleep because it was only collected at ages 5,7,9 and 11. Other studies in Germany, Canada and China have also established that reduced sleep duration at childhood is an obesity risk factor.
- Adequate sleep and obesity in adults
Unlike in children, there are fewer studies indicating a correlation between reduced sleep and obesity. In addition, most of the studies on the subject have been cross-sectional rather than the more reliable longitudinal studies. Findings of longitudinal studies involving adults have been less consistent than those involving children. One such studies followed 68,000 middle-aged American women for 16 years to establish if there was a correlation between their sleep duration and their weight. The study found that those who slept for 5 hours or less had a 15% probability of becoming obese compared to those who slept for 7 hours (Patel, Malhotra, White, Gottlieb, & Hu, 2006). The strength of this study is in large sample size and its longitudinal approach; as earlier mentioned there are very few adult longitudinal studies on the subject at hand. The limitations include the fact that the data was collected using questionnaire thus liable to misclassification. Secondly the subjects were only middle-aged women in the nursing career and mainly Caucasian. As such, generalizing the findings of the study to other groups such as men and other age groups. Another similar study involving 177,000 female nurses sought to establish if working longer rotating shifts predisposed the participants to obesity, cardiovascular diseases, cancer and diabetes. The participants were followed up for 18-20 years and the effects of the longer rotating working hours documented. The strength of the study was in the large sample size of about women (the largest reviewed in this paper) and long-term followup. The limitations include the fact that the study sample was mainly made of white female nurses and thus the generalizability of the data to other groups in population is limited. The second limitation is in the fact that the data on rotating night shifts was self-reported and thus accuracy could not be guaranteed. A few other studies have found that long sleep durations are linked to higher risk of obesity .
However, the discussed studies are all observational and thus cannot be conclusively used to show that adequate sleep lower risk of obesity as only randomized clinical trials are used to make definite conclusions on efficacy of an intervention. Unfortunately, only one such study has been conducted but two trials are in progress, in New Zealand and US. In Australia, a study involving 3287 7-month-old infants with sleep problems, but had no conclusive findings due to subjects dropping out of the study and other serious shortcoming
6.0 Mechanisms of reduced sleep as an obesity risk factor
The exact mechanisms of how sleep affects body weight remain unclear. However, scholars have made several speculations on how sleep reduction may cause weight gain by increasing food intake or decreasing energy burnt (Patel & Hu, 2008). The most common theory on how sleep deprivation can lead to weight gain is the alteration of the production of hormones that control the appetite. Studies have found that reduced sleep is associated with higher levels of appetite-stimulating hormone (ghrelin) and lower levels of satiety inducing hormone (leptin). This means that sleep deprived persons have hunger pangs and an appetite, particularly for fat and carbohydrate foods (Patel, 2009 ; Bell & Zimmerman, 2010). As the sleep deprived people consume more fats and carbohydrates, the level of fat adiposity increases leading to obesity. A recent study found that sleep deprived people had a higher affinity for high calorie foods and drinks, may be in a bid to compensate for the apparent energy burnt during the prolonged working hours. Reduced sleep also gives people more time to eat, and they often choose less healthy diets. Last but not least, reduced sleep has the effect of reducing physical activity due to tiredness during the day (Patel, Malhotra, White, Gottlieb, & Hu, 2006).
- Conclusion
While several risk factors have been associated with the increasing prevalence of obesity, the parallel increase in obesity prevalence and sleep reduction has attracted a lot of research interest leading to a growing body of knowledge indicating that reduced sleep is an obesity risk factor. The correlation has been established with more certainty in children than in adults. The exact mechanisms of reduced sleep as a risk factor for obesity remain unknown but there are several speculations that continue to be investigated.
References
Bayer, O., Rosario, A., Wabitsch, M., & von Kries, R. (2009). Sleep duration and obesity in children: is the association dependent on age and choice of the outcome parameter? Sleep , 32 (9), 1183-9.
Bell, J. F., & Zimmerman, F. J. (2010). Shortened Nighttime Sleep Duration in Early Life and Subsequent Childhood Obesity. Journal of American Medical Association,Pediatrics , 164 (9), 840-845.
Chaput, J., Brunet, M., & Tremblay, A. (2006). Relationship between short sleeping hours and childhood overweight/obesity: results from the 'Québec en Forme' Project. International Journal of Obesity , 30 (7), 1080-5.
Chen, M., Wang, E., & Jeng, Y. (2006). Adequate sleep among adolescents is positively associated with health status and health-related behaviors. BMC Public Health , 8 (6), 59.
Gillman, M., Rifas-Shiman, S., Kleinman, K., Oken, E., Rich-Edwards, J., & Taveras, E. (2008). Developmental origins of childhood overweight: potential public health impact. Obesity , 16 (7), 1651-6.
Greer, S. M., Goldstein, A. N., & Walker, M. P. (2013). The impact of sleep deprivation on food desire in the human brain. Nature Communications , 4 (2259).
Jiang, F., Zhu, S., Yan, C., Jin, X., Bandla, H., & Shen, X. (2009). Sleep and obesity in preschool children. The Journal of Pediatrics , 154 (6), 814-8.
Landhuis, C., Poulton, R., Welch, D., & Hancox, R. (2008). Childhood sleep time and long-term risk for obesity. Pediatrics , 122 (5), 955-60.
Nielsen, L., Danielsen, K., & Sorensen, T. (2011). Short sleep duration as a possible cause of obesity: critical analysis of the epidemiological evidence. Obesity Reviews , 12 (1), 78-92.
Pan, A., Schernhammer, E., Sun, Q., & Hu, F. (2011). Rotating night shift work and risk of type 2 diabetes: two prospective cohort studies in women. PLoS Med , 8 (12).
Patel, S. R., Malhotra, A., White, D. P., Gottlieb, D. J., & Hu, F. B. (2006). Association between Reduced Sleep and Weight Gain in Women. American Journal of Epidemiology , 164 (10), 947-954.
Reilly, J., Armstron, J., Dorosty, A., Emmett, P., Ness, A., Rogers, I., et al. (2005). Early life risk factors for obesity in childhood: cohort study. BMJ; Clinical reserach education , 330 (7504), 1357.
Taveras, E., Rifas-Shiman, S., Oken, E., Gunderson, E., & Gillman, M. (2008). Short sleep duration in infancy and risk of childhood overweight. Archives of pediatrics and adolescent medicine , 162 (4), 305-11.
Wake, M., Price, A., Clifford, S., Ukoumunne, O., & Hiscock, H. (2011). Does an intervention that improves infant sleep also improve overweight at age 6? Follow-up of a randomised trial. Archives of Disease in Childhood , 96 (6), 526-32.
