Response to Dr Andersen et al.

We are grateful to the colleagues from the Federal University of São Paulo and Marilia Medical School, Brazil, for their suggestion to investigate sleep duration in future studies.¹

However, the association between sleep duration and obesity has not been fully understood and supported by prospective epidemiological studies.^{2, 3} Cappuccio et al.⁴ based on a meta-analysis involving 30 002 children and 604 509 adults did not confirm a causal inference between sleep disturbances and obesity due to lack of control of important confounders and inconsistent evidence of temporal sequence in prospective studies. The relationship was confirmed in cross-sectional studies, although a large representative sample of 102 353 US children from the National Survey of Children’s Health failed to reveal any independent association between insufficient sleep and childhood obesity.⁵

The relationship among sleep, sedentary behavior and hormonal levels should be assessed in detail, in prospective studies, to understand how those factors may affect obesity. Moreover, Touchette et al.⁶ recommends investigation on valid measurement tools to accurately seize duration as well as quality of sleep.

According to Must and Parisi,³ further delineation of the complex interrelation of sleep, time use, mood, and familial aspects in racially and economically diverse samples are needed before sleep interventions can be designed or preventive guidance offered.

On the other hand, Kim et al.⁷ argued that obstructive sleep apnea syndrome (OSAS) is an increasingly common health problem in children, and in the last decade, the emergence of increasing obesity rates has further led to remarkable increases in the prevalence of OSAS. Therefore, in a review, the author explored potential interactions between OSAS and obesity suggesting that this important health problem in children can also mediate the relationship between sleep duration and obesity and should be investigated.

Apparently, several factors may interfere on sleep duration with a probable impact on child obesity. Among those factors, TV, mobile phones, electronic devices such as computers, video games and other electronic gadgetry, particularly in the bedroom of the children, can interfere with their sleep duration.^{8, 9, 10, 11} Nevertheless, the children included in the study by Sacco et al.¹² are from Capão Bonito, a small town in the poorest region of São Paulo state, near Ribeira valley, where one in every four people live in rural areas and children have enough space to play outside home, with not much access to electronic media.

Variables related to antenatal care, as suggested by the colleagues, were investigated by Sacco et al., as well as data regarding breastfeeding (shown in Table 1) and actual food intake (assessed by a food frequency questionnaire (FFQ)). Only 0.8% of the women included in the study did not take antenatal care. Breastfeeding was considered as a confounder, and did not show statistically significant associations with the outcomes of interest, as has been emphasized by Sacco et al.¹² Data from the FFQ have been analyzed, although these data are not related to the objective of the present study.

Table 1 Characteristics of the children and respective mothers (n=98)

In summary, we believe that the relationship between sleep duration/quality and obesity is an important area for research, and we congratulate the colleagues for their choice.