Article In Brief
New findings suggest that among toddlers from low-income families inconsistent sleep schedules were linked with an increased body mass index. The study sheds light on the link between poverty, poor sleep, and obesity.
Inconsistent sleep schedules among toddlers in low-income families were associated with higher body-mass index, even after physical activity, diet quality, and sleep duration were factored in, according to a study in Annals of Behavioral Medicine.
The findings suggest that the consistency of sleep onset could be an important mediator between childhood poverty and obesity, the researchers wrote. The study should spark increased attention to the regularity of sleep onset, they said.
“Those who have expertise in sleep and sleep hygiene and sleep physiology should look at this,” said Maureen Black, PhD, professor of pediatrics at the University of Maryland School of Medicine, who helped lead the study along with Lauren Covington, PhD, MS, RN, assistant professor at the University of Delaware School of Nursing.
“Researchers should look beyond sleep duration, to consider the regularity of sleep onset, particularly among low-income families,” Dr. Covington said. It is recommended that children go to bed within an hour of their usual bedtime on a nightly basis. But this scheduling may be difficult though, for families living in poverty, she said, where a parent may face challenges such as being the sole caregiver of several children or working multiple jobs and non-standard work schedules.
Independent experts said the study highlights an important message, but pointed to relevant data that was not included or considered, making it challenging to interpret the findings.
Study Design, Findings
In the study, researchers drew data from a randomized controlled trial on obesity prevention, in which 277 pairs of mothers and their toddlers, age 12 to 32 months, were recruited from a suburban Special Supplemental Nutrition Program for Women, Infants and Children clinic and an urban pediatric clinic, both of which serve low-income communities.
For the trial, investigators placed small, lightweight accelerometers on the ankles of the mothers and toddlers, helping them track moderate to vigorous physical activity, as well as sleep onset and sleep duration, using an algorithm. They wore the devices for about seven consecutive days; those who wore them for fewer than three days were not included in the analysis. They considered a sleep period at least 15 minutes of sleep coming after at least one minute of awake time and before at least five minutes of awake time.
Researchers measured weight and recumbent length to calculate BMI-for-age z scores according to the World Health Organization growth charts. [BMI z scores, also called BMI standard deviation scores, are measures of relative weight adjusted for child age and sex.] Data on the toddlers' diet was collected in interviews with their mothers and researchers assessed diet quality using the Healthy Eating Index-2015, which determines how well food choices align with key recommendations of the Dietary Guidelines for Americans. They assessed the household poverty ratio using parent-reported income and family size based on thresholds established by the US Census Bureau.
The researchers found that the consistency of sleep onset mediated the relationship between poverty and BMI z scores, with greater consistency predicting a lower BMI z score (effect size -6.822; 95% CI -13.936--1.274). Neither diet quality nor the amount of moderate to vigorous physical activity mediated that relationship, the researchers reported.
That sleep onset played this role, seemingly independent of diet and physical activity, is a surprising finding, Dr. Black said. One theory that could explain the role that sleep onset plays in childhood weight is the “structured days hypothesis,” that the more structured a child's day is—with sleeping times, eating and activities built into a regular schedule—the healthier children will eat and the more exercise they'll get.
But these findings suggest there's more to it than that, Dr. Black said.
“What we find is that even accounting for other factors that do contribute to obesity—how much you eat, how physically active you are—this regularity of sleep has an association with BMI z-score. What it suggests is that there may be additional factors associated with sleep onset that are related to weight gain, such as the Circadian and Circannual Rhythm Model or other biological or regulatory processes.” Helping young children develop a consistent sleep schedule may reduce excess weight gain and support healthier lifestyle behaviors, she said.
Jane Fan, MD, a professor of child neurology who specializes in sleep medicine at the University of North Carolina School of Medicine, said drawing attention to the importance of consistent sleep onset and bedtime in toddlers and young children is a positive message.
“The good thing about this study is that the conclusion is a good and important one,” she said. “The parents need to put their babies to bed at a regular time. That's so important. For some of the families, a consistent bedtime may not be straightforward.”
Sleep onset time is mostly determined by bedtime and sleep onset latency. In toddlers, the average sleep onset latency is about 20 minutes (10-30 minutes) in children without co-existing medical conditions or behavioral insomnia, Dr. Fan said. Certain medical conditions, such as acid reflux or sleep apnea, if untreated, could delay sleep onset, as well, she noted.
She pointed to several data points, however, that complicate the interpretation of the link between sleep onset and BMI z-score. Since normal BMI varies with age and gender, a BMI z score often is used to assess obesity. A score of two or higher indicates the weight is more than 97 percent compared to population data from the Centers for Disease Control and Prevention (CDC). Obesity in this age group using BMI z scores would be about two or more, she said. In these toddlers, the mean BMI z score was just 0.6, which she said is “within the normal range.”
“I have not seen the data related to what percentage of the subjects is in the obese category,” she said. “If sleep inconsistency is correlated with a higher BMI z score in the obese group (subjects with BMI z score>= two), then that would be a more convincing conclusion.”
She also noted that wake-up times or get-up times are not included, even though they are important data when considering a likely mechanism at work—the effects on circadian rhythm.
Dr. Fan noted that the nighttime sleep durations of an average of between seven hours, 18 minutes and eight hours, 17 minutes, depending on the time point measured, was “very short for a 2-year-old.”
Insufficient sleep duration exerts a more negative impact on a young developing brain than on a mature brain, she said. Measuring sleep time over just one week was a fairly short time frame over which to try to assess consistency. Some of the children had sleep measured over just three nights.
Dr. Fan said she makes inquiring about bedtime and sleep onset time a part of history in clinical visits, and when she finds this schedule is inconsistent, she recommends a consistent bedtime, routines for quieting down before bedtime, avoiding bottles at night to avoid acid reflux and an appropriate bedroom setting that minimizes allergens and has low light and a cool temperature.
Temitayo Oyegbile-Chidi, MD, PhD, an associate professor of neurology at the University of California, Davis, where she specializes in pediatric and adult sleep medicine, agreed that the message of the study was worthwhile and it made some valid points.
“Multiple factors play a role in BMI. Sleep health is one of them and most importantly, consistency in bedtimes can lead to improved sleep health,” Dr. Oyegbile-Chidi said.
She said that while the authors mention several possible contributors to BMI and even make an attempt at directionality, it is difficult to determine directionality easily since most of these factors are likely bi-directional.
She also noted that the sample was 68 percent Black. The role that race and racism should also be considered since racism is a major social determinant of health and because this study sample was not very heterogenous, she said. “Studies have shown that maternal/familial experiences of racial discrimination can play a role in sleep and weight of offspring during the first few years of the offspring's life. So it is certainly a factor that must be considered,” Dr. Oyegbile-Chidii said.
For toddlers, she added, it's important to underscore the necessity of consistency for adequate sleep health and overall well-being. As early as toddlerhood, children can develop good sleep patterns and sleep habits that can shape their lives and health long-term, she said.
“Optimization of nighttime sleep will improve daytime function so that young children and toddlers can focus on their learning, be less likely to be hyperactive during the day—which can be a symptom of poor sleep in childhood—and be more likely to exercise and stay active because they are awake and alert.”
She said part of the value of this paper is the age group studied.
“This study certainly begins to establish the relationship between sleep and BMI and the longitudinal effects overtime in toddlers, which is an age group that we have focused on less. We've looked at these relationships in adults or older children, but studying these effects in toddlers begins to address that notable gap in the literature and the findings are very enlightening.”
Drs. Covington, Fan, Oyegbile-Chidi, and Black had no disclosures.