Sleep duration of underserved minority children in a cross-sectional study

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1 Wong et l. BMC Pulic Helth 2013, 13:648 RESEARCH ARTICLE Open Access Sleep durtion of underserved minority children in cross-sectionl study Willim W Wong 1*, Christin L Ortiz 2, Der Lthn 2, Louis A Moore 2, Kren L Konzelmnn 1, Anne L Adolph 1, EO Brin Smith 1 nd Nncy F Butte 1 Astrct Bckground: Short sleep durtion hs een shown to ssocite with incresed risk of oesity. Childhood oesity is more prevlent mong underserved minority children. The study mesured the sleep durtion of underserved minority children living in lrge US urn environment using ccelerometry nd its reltionship with BMI, socioeconomic sttus (SES), gender, ethnicity nd physicl ctivity. Methods: Time spent on sleep nd physicl ctivity mong 333 Hispnic nd 150 lck children (9 12 y) ws mesured ojectively y ccelerometry over 5 7 consecutive dys. The children were recruited t 14 underserved community centers in Houston, Texs, etween Jnury 2009 nd Ferury Body weight nd height were mesured in duplicte. Results: The mjority of children (88.8%) wore the monitor for 6 consecutive dys. The children slept 8.8 ± 0.6 (men ± SD) h/d nd spent 45 ± 24 min/d on moderte-vigorous physicl ctivity (MVPA). Hispnic children slept 0.2 h/d longer (P < 0.001) thn lck children. Oese children slept 0.2 h/d less (P < 0.02) thn norml-weight children. SES hd no effect on sleep durtion. There ws significnt interction etween gender nd ge (P < 0.03); girls ged y slept 0.3 h/d less thn oys nd the younger girls. Children slept 0.6 h/d longer (P < 0.001) during the weekend thn weekdys. No reltion ws detected etween sleep durtion nd MVPA time. Conclusions: Minority children living in lrge metropolitn re in the US re not meeting the Ntionl Sleep Foundtion recommendtion for sleep durtion of h/d. Longitudinl studies sed on ojective mesures re needed to estlish cuslity etween sleep durtion nd oesity risk mong minority children. Keywords: Sleep, Minority children, Oesity Bckground The prevlence of sleep disturnces hs incresed drmticlly, concurrent with the oesity epidemic in the US [1]. The Ntionl Sleep Foundtion recommends tht school-ge children should sleep h/d [2]. The recommendtion ws supported y oservtionl dt from two lrge cross-sectionl studies involving thousnds of children nd met-nlysis of sleep dt sed on 12 studies with 30,002 children ged 3 18 y from Frnce, Tunisi, Jpn, Germny, USA, Brzil, Portugl, United Kingdom, Cnd, Tiwn nd Chin [3-5]. Numerous studies lso hve documented tht short sleep durtion is ssocited with incresed risk of childhood nd dult oesity [3,4,6-10]. In n dult sleep det study, short sleep durtion led to decrese in serum leptin nd n increse in ghrelin suggesting tht short sleep might stimulte ppetite nd increse food intke [11,12]. A recent study on 30 helthy dults lso showed tht sleep restriction led to incresed ctivtion of rin regions tht re sensitive to food stimuli [13]. The ltest Ntionl Helth nd Nutrition Exmintion Survey showed tht the prevlence of overweight nd oesity ws highest mong Hispnic oys (39.6% overweight, 23.4% oese) nd lck girls (41.3% overweight, 24.3% oese), ges 2 19 y [14]. The sleep * Correspondence: wwong@cm.edu 1 USDA/ARS Children s Nutrition Reserch Center, Deprtment of Peditrics, Bylor College of Medicine, Houston, Texs, USA Full list of uthor informtion is ville t the end of the rticle 2013 Wong et l.; licensee BioMed Centrl Ltd. This is n Open Access rticle distriuted under the terms of the Cretive Commons Attriution License ( which permits unrestricted use, distriution, nd reproduction in ny medium, provided the originl work is properly cited.

2 Wong et l. BMC Pulic Helth 2013, 13:648 Pge 2 of 7 durtion mong underserved minority children is not well documented. This study descries sleep durtion nd its ssocition with BMI, socioeconomic sttus (SES), gender, ethnicity nd physicl ctivity in lrge smple of underserved minority children living in n urn environment using n ojective instrument, the Acticl ccelerometer. Methods Study popultion A totl of 483 minority children ged 9 12 y, regrdless of ody weight, with no physicl or medicl limittions to free ply, nd living in the Greter Houston Metropolitn re were enrolled in the Helthy Kids-Houston Study etween Jnury 2009 nd Ferury The Helthy Kids-Houston is community-sed fterschool progrm to promote helthy lifestyle mong minority children. Ech progrm consists of three 6-week sessions, once in the fll, once in the spring nd once t the end of the school yer. The children were recruited from economiclly distressed neighorhoods surrounding 14 community centers mnged y the City of Houston Prks nd Recretion Deprtment (HPARD) nd re representtive of minority children living in the Greter Houston Metropolitn Are. The progrm ws promoted through newsletters of the community centers nd t nery pulic schools. These schools were populted primrily y minority children with the mjority of them qulified for free or reduced cost school mel progrms. The dt collection ws done over two yers due to limited physicl cpcity of the community centers nd to insure sufficient stff for progrm implementtion nd child sfety. The study protocol ws pproved y the Institutionl Review Bord for Humn Suject Reserch for Bylor College of Medicine (BCM) nd Affilited Hospitls. To enroll in the study, the prents completed the HPARD enrollment form nd the BCM consent form. The progrm rochures, enrollment forms, nd consent forms were ville in oth English nd Spnish. Weight nd height Body weight nd height were mesured in duplicte with n electronic digitl scle nd with digitl stdiometer to the nerest 0.1 kg nd 0.1 cm, respectively. The electronic scle nd stdiometer were clirted with reference weight nd rod, respectively, prior to the mesurements. The mesurements were mde y project stff who hd received trining on the proper mesurement procedures nd the proper use of the equipment. Children were considered oese if their BMI vlues were 95 th percentile, overweight if their BMI vlues were 85 th percentile ut < 95 th percentile, nd norml-weight if their BMI vlues were 5 th percentile ut < 85 th percentile. Sleep durtion Acticl (Philips Respironics, Bend, OR) ccelerometersed monitors were used to ojectively mesure sleep durtion [15]. The monitors were ffixed ove the ilic crest of the right hip with n elstic elt nd djustle uckle. Children were instructed to wer the ctivity monitor for seven consecutive dys, nd to remove the monitor only for thing or swimming. A log recording the times nd resons for monitor removl, np nd night sleep times ws kept y the children nd prents. Night sleep durtion ws determined from the 24-h ccelerometer mesurements y visul inspection y single qulified technicin in order to minimize interinterpreter vrition. A plot of ctivity counts per minute (cpm) for ech 24-h period ws used to identify the time of sleep onset nd termintion. The onset of night sleep ws identified y run-in period of consecutive zeros, followed y longer period, usully lsting severl hours, of consecutive zeros with occsionl minute vlues with counts <200 cpm, indictive of movement during sleep. The termintion of night sleep ws identified y n rupt increse in ctivity counts >200 cpm tht lsted severl minutes, indictive of wkening nd getting out of ed. Any minutes scored >200 cpm during the night were considered wke nd were removed from the sleep durtion. The ctivity logs were used to confirm night sleep nd to identify nps, which were uncommon in this group of children. Physicl ctivities The mount of time the children spent on sedentry, light nd moderte-vigorous physicl ctivities ws lso extrcted from the Acticl dt [15]. Sedentry level ws defined s ctivity energy expenditure (AEE) <0.01 kcl. kg -1. min -1 or physicl ctivity rtio (PAR) < 1.5, encompssing physicl ctivities of miniml ody movements in the sitting or reclined position. Light level ws set t 0.01 < AEE < 0.04 kcl. kg -1. min -1 or 1.5 < PAR < 3.0, reflective of low level of exertion in the stnding position. Moderte level ws set t 0.04 < AEE <0.10 kcl. kg -1. min -1 or 3.0 < PAR < 6.0, nd involved medium exertion in the stnding position. Vigorous level ws set t AEE > 0.10 kcl. kg -1. min -1 or PAR > 6.0, reflective of ctivities t high level of exertion in the stnding position. Socio economic sttus (SES) The SES of the children ws sed on informtion provided y the prents in the progrm enrollment form. Children were considered to e from low socioeconomic fmilies when they qulified for free or reduced cost mel progrm t school nd qulified for federl or stte medicl insurnce progrms for low-income fmilies.

3 Wong et l. BMC Pulic Helth 2013, 13:648 Pge 3 of 7 Sttisticl procedures Descriptive sttistics were used to generte the mens nd stndrd devitions of ll the outcome mesures. Independent-smples t-test or Person Chi-Squre test ws used to evlute potentil differences etween the rce/ethnicity groups. Generlized liner models including ll 2- nd 3-wy interctions were used to test the effects of gender, ge (9 10 y vs y), rce/ethnicity (lck vs. Hispnic), oesity sttus (norml-weight, overweight, oese) nd SES on sleep durtion with nd without community centers in the model. Non-significnt 2- nd 3-wy interctions, strting with the most nonsignificnt 3-wy interction, were removed from the model one t time using ckwrd stepwise elimintion procedure. The sme sttisticl procedure ws used to exmine the reltionship etween sleep durtion nd physicl ctivity. Generlized estimting equtions for repeted mesures ws used to test the potentil differences in sleep durtion etween weekdys nd weekend dys with djustment for ge, gender, rce/ethnicity, oesity sttus nd community center. Sttisticl nlyses were performed with SPSS softwre (version 20, SPSS Inc, Chicgo, IL). An lph vlue of 0.05 ws used in ll sttisticl nlyses. Results Tle 1 descries the demogrphic nd physicl chrcteristics of the 483 children. Age, gender distriution, nd ody weight did not differ y rce/ethnicity (P = 0.35). The lck children were tller (P = 0.001) nd the Hispnicchildrenwerehevieryodymssindex(BMI, Tle 1 Demogrphic nd physicl chrcteristics of study sujects Vriles All children Hispnics Blck N Age, y 10.3 ± ± ± 0.99 Gender (M/F) 247/ /158 72/78 Weight, kg 47.6 ± ± ± 1 Height, cm ± ± ± 9.2* BMI, kg/m ± ± ± 7.1** BMI z-score 1.32 ± ± ± 1.18** Weight sttus, N (%) Norml 164 (34.0) 77 (23.1) 87 (58)** Overweight 84 (17.4) 66 (19.8) 18 (12.0)** Oese 235 (48.7) 190 (57.1) 45 (30.0)** Low SES (%) c * Hispnic nd lck groups were compred using independent-smples t-test for continuous vriles nd Person Chi-Squre for ctegoricl vriles. Significnt differences etween Hispnic nd lck re indicted s follows: * for P < 0.01 nd ** for P < Men ± SD. c SES = socio economic sttus. Bsed on qulifiction for free/reduced cost mels t school nd Medicid. P < 0.001) nd BMI z-score (P < 0.001). The mjority of children (66.1%) were overweight or oese with more Hispnic children flling into these ctegories (76.9% vs. 42.0%, P < 0.001) thn lck. Approximtely 93% of the children were from low-income fmilies with more Hispnic children flling into the low SES ctegory (P < 0.006) thn lck. The mjority of children (88.8%) wore the Acticl for t lest 6 consecutive dys while 72.9% wore the Acticl for 7 consecutive dys. Figure 1 shows the distriution of sleep durtion recorded for ll the children. The children slept n verge of 8.8 ± 0.6 h/d (Prediction Intervls: h/d). Among the 483 children, only 12 children slept 10 hours or longer. Figure 2 shows the effects of gender, ge, rce/ethnicity, oesity sttus, SES, nd weekdy/weekend on sleep durtion sed on generlized liner model. A significnt interction ws detected etween gender nd ge (P = 0.03). As shown in Figure 2A, the older girls (11 12 y) slept less ( 0.2 to 0.3 h/d, P < 0.03) thn the younger children (9 10 y). The Hispnic children slept more (+0.2 h/d, P <0.001) thn the lck children (Figure 2B). Oese children slept less ( 0.2 h/d, P = 0.02) thn the normlweight children (Figure 2C). SES hd no effect on sleep durtion (Figure 2D, P = 0.26). Inclusion of community center in the nlysis did not chnge the outcomes s reported in Figures 2A, B, C nd D. Figure 2E shows tht children slept more (+0.6 h/d, P < 0.001) during weekend thn weekdys. The results remined unchnged with the inclusion of ge, gender, rce/ethnicity, oesity sttus nd community center in the model. When oesity sttus ws replced y BMI z-scores in the generlized liner model, sleep durtion ws reducedy0.1h/d(p=0.001)withechunitincresein BMI z-score. Since the mjority of the children (93%) were from low-income fmilies, the generlized liner Numer of children Men ± SD = 8.8 ± 0.6 h/d (Prediction Intervls: h/d) Rnge: h/d Sleep durtion (h/d) Figure 1 Frequency distriution of sleep durtion in 483 underserved minority children. The histogrm represents the sleep durtion per dy for the children in given intervl.

4 Sleep durtion (h/d) (A) N = 181 N = 66 N = 183, N = 53 ) ) y) y) 2y 0y (9( s( ys s( rls l i y o r G B Gi Bo (C) N = 164 N = 84, N = 235 (B) N = 333 N = 150 Hispnic (D) N = 33 N = 450 High SES Low SES Norml Overweight Oese Sleep durtion (h/d) Pge 4 of 7 Blck Sleep durtion (h/d) Sleep durtion (h/d) Sleep durtion (h/d) Wong et l. BMC Pulic Helth 2013, 13:648 (E) N = 479 N = 483 Weekdys Weekend dys Figure 2 Estimted sleep durtion (h/d). (A) gender nd ge: Boys 9 10 y, n = 181; Boys y, n = 66; Girls 9 10 y, n = 183; Girls y, n = 53. (B) rce/ethnicity: Blck, n = 150; Hispnic, n = 333. (C) oesity sttus: Norml, n = 164; Overweight, n = 84; Oese, n = 235. (D) socioeconomic sttus (SES): High SES, n = 33; Low SES, n = 450. (E) weekend/weekdy: Weekdys, n = 483; Weekend dys, n = 479. The whisker represents the stndrd error of the men. Different letters (, ) ove the columns in ech figure indicte significnt difference. model nlysis ws repeted without djusting for SES. Similr results were otined for rce/ethnicity (Hispnic children slept 0.3 h/d longer thn lck children, P < 0.001), oesity sttus (oese children slept 0.2 h/d less thn norml-weight children, P < 0.01), nd gender/ ge (older girls slept 0.3 h/d less thn the younger girls nd oys, P < 0.02). Similr results lso were otined when we limited the sttisticl nlysis on the children who wore the Acticl for 7 consecutive dys. Overll, the children spent 528 ± 63 min/d on sedentry physicl ctivity, 281 ± 57 min/d on light physicl ctivity ut only 45 ± 24 min/d on moderte-vigorous physicl ctivity (MVPA). The mount of time the children spent on MVPA ws less thn the US pulic helth recommended level of 60 min/d [16]. Generlized liner models nlysis with djustment for ge, gender, rce/ ethnicity, oesity sttus nd center showed no reltionship etween MVPA nd sleep durtion (P = 0.27). The sme nlysis, however, showed significnt negtive reltionship etween sedentry/light physicl ctivities nd sleep durtion (P < 0.01) Discussion The mjority of the sleep studies in children re sed on sujective reports from prents. Only four studies reported the use of ccelerometry to mesure sleep durtion in children [17-20]. The study reported y Gupt ws done on dolescent girls ged y sed on 24-h of wrist ctigrphy [19]. Benefice reported the sleep durtion on Seneglese mlnourished dolescent girls ged 13.3 y sed on 3 4 d of ccelerometry mesurements [20]. Nixon mesured the sleep durtion on 7-y-old children y wrist ctigrphy [18]. The most recent study mesured the sleep durtion of 308 children ged 4 10 y using wrist ctigrphy over one week with the mjority (71.4%) of the children eing white [17]. By convention, wrist ctigrphy hs een used in sleep studies. However, s demonstrted in recent puliction compring sleep durtion mesured y wrist- or hip-ccelerometers in y old children, oth ccelerometers yielded identicl results nd the difference of 6.8 minutes ws considered to e cliniclly non-significnt [21]. The ccurcy of sleep durtion mesured y Acticl ccelerometers is further supported in seprte study on 30 dolescents who wore three ctigrphs (Acticl, Sleepwtch nd Actiwtch) concurrently while their sleep durtion ws ssessed y the reference method, polysomnogrphy [22]. The study showed tht Acticl yielded similr sleep durtion when compred to Sleepwtch nd Actiwtch. Interestingly, ll three ctigrphs overestimted sleep durtion y min/d when compred to polysomnogrphy during single night mesurement. The uthors indicted tht the difference might e due to the wke-time sensitivity etween ctigrph nd polysomnogrphy. However, if the overestimtion of sleep durtion is correct, we would hve overestimted the sleep durtion of the underserved minority children nd therefore they would e in further sleep deficit. The greement in sleep durtion mesurements etween Acticl nd Actiwtch hs lso een demonstrted in 20 young dults who wore oth monitors for 5 9 consecutive dys [23]. Polysomnogrphy certinly is the reference method to mesure sleep durtion. However, the procedure is expensive nd invsive ecuse it requires the prticipnts to sleep in sleep lortory nd therefore removing them from their nturl sleeping settings. Furthermore, polysomnogrphy will most likely limited the mesurement to single night wheres ccelerometers such s Acticl will llow mesurement of sleep durtion over period of severl dys nd therefore provide more representtive mesurement of sleep durtion. It lso will e difficult to implement the polysomnogrphy procedure in study involving lrge smples of children. Therefore this study represents the first ojective mesurement of sleep durtion over period of 5 7 d of lrge smple of underserved minority children ged 9 12 y living in lrge US urn nd economiclly distressed environment.

5 Wong et l. BMC Pulic Helth 2013, 13:648 Pge 5 of 7 Our results showed tht these underserved minority children were not meeting the Ntionl Sleep Foundtion recommended sleep durtion of h/d for school-ged children [2]. Our results (Figure 2A) showed tht older girls slept less thn the younger children nd tht lck children lso slept less thn Hispnic children (Figure 2B). If short sleep durtion is risk fctor for childhood oesity, older lck girls should e encourged to sleep longer ecuse they hve een documented to hve higher prevlence of overweight nd oesity thn lck oys nd Hispnic girls [14]. Since oese children slept less thn norml-weight children (Figure 2C), prents should encourge their children to go to sleep erlier or to provide home environment to encourge uninterrupted sleep. We found no ssocition etween SES nd sleep durtion (Figure 2D). Since the project trgeted the underserved communities, the lck of ssocition etween SES nd sleep durtion is proly ttriutle to the smll numer of children (n = 33) in the high SES ctegory. As shown erlier, excluding SES in the nlysis did not ffect the effects of BMI, rce/ethnicity nd ge/gender on sleep durtion. In Houston, Hispnics nd lcks ccount for ~62% of the popultion. More importntly, Hispnics nd lcks constitute t lest 30% of the popultion in the United Sttes most populous sttes such s Cliforni, Texs, New York, Florid, Illinois, Arizon, New Mexico, New Jersey, Nevd nd North Crolin. Since these sttes ccounted for lmost 50% of the totl U.S. popultion, oesity-relted diseses, prticulrly mong the minority popultions, will hve mjor negtive impct on the locl, stte, nd federl helthcre system. Our sleep dt showed tht minority children, including the normlweight children, were not meeting the recommended sleep durtion of h/d. Our results lso showed tht oth BMI z-scores nd oesity sttus of the minority children were negtively ssocited (P < 0.01) with sleep durtion suggesting tht longer sleep durtion might led to n improvement in BMI z-scores nd oesity sttus. Our cross-sectionl dt cnnot sustntite the cusl reltionship which would require longitudinl oservtions. Our results lso showed tht only 12 of the 483 minority children were sleeping 10 h/d or longer. With most minority popultions living in the mjor U.S. cities, it is likely tht the demogrphic profile of the minority popultion living in Houston, Texs might e similr to those living in the other mjor metropolitn cities. If lck of sleep is risk fctor for childhood oesity, fmilies should encourge their children to go to ed erlier in order to meet the sleep recommendtion. In the most recent ntionl survey in the United Sttes [14], the prevlence of overweight nd oesity remined significntly elevted mong minority children when compred to white children. The survey showed tht pproximtely 25% of white children were overweight nd etween 6% nd 15% were oese depending on their ges. A recent study mesured the sleep durtion on 308 children (7.2 ± 1.3 y) over 1-wk period using wrist ccelerometry nd found tht these children slept n verge of 8 h/d [17]. Since the mjority of the children (71.4%) in the study ws white, together with our dt on minority children, one cn speculte tht children living in the United Sttes, regrdless of rce/ethnicity, might not e meeting the Ntionl Sleep Foundtion recommendtion of h/d of sleep. There is possiility tht children might tke dytime nps nd compenste for the short sleep durtion, prticulrly mong the Hispnic children. However, children etween 9 nd 12 yers of ge seldom tke dytime nps, prticulrly during school dys. In n unrelted study of 4,470 Chinese children living in Hong Kong with n verge ge of 9.2 ± 1.8 y [24], only pproximtely 6% of the children took nps. Therefore, it is not likely tht dytime nps mong our underserved minority children will compenste for their short sleep durtion. In cross-sectionl study of 5,159 Chinese children with n verge ge of 9.3 ± 1.8 y, children who slept more during the weekend hd lower risk of overweight nd oesity [25]. The sleep dt were collected using 54-item sleep questionnire nd the nthropometric dt were sed on prent reports. When weekend sleep compenstion ws included in the generlized liner models, we found no compenstion effect on BMI z- scores or oesity sttus (P 0.18). The much lrger smple size in the study of Chinese children might increse the power to detect the effect of sleep compenstion on oesity risk. It is lso possile tht there is significnt rce/ethnicity difference in sleep compenstion etween Chinese children nd Hispnic nd lck children. To evlute the potentil compenstion effect, much lrger sleep study mong underserved minority children is needed. However, if the sleep compenstion result reported mong the Chinese children is true nd pplicle to other rce/ethnic groups, the result indirectly suggested tht the h/d sleep durtion recommended y the Ntionl Sleep Foundtion might e pproprite. In study crried out in Hong Kong, leisure extrcurriculr ctivities nd lter school strt time were found to lengthen sleep time mong school-ged children [24]. However, high socioeconomic sttus, medi use nd homework were found to shorten sleep time. In seprte study mong dolescents [26], dely of school strt time y 30 minutes led to n increse in sleep time y 45 minutes; more stisfction with sleep; improved motivtion; reduced sleepiness, ftigue nd depressed mood; nd improved clss ttendnce. Both studies gin

6 Wong et l. BMC Pulic Helth 2013, 13:648 Pge 6 of 7 suggested tht longer sleep durtion might hve mny eneficil effects including reducing the risk of oesity. The eneficil effects y delying school strt time re very interesting nd need to e confirmed, prticulrly mong underserved minority children. With schools pushing for etter cdemic outcomes, children re often given more homework. If the negtive effect of homework on sleep time is rel, it will e chllenge for the schools to lnce etween school hours nd homework lod. With the redy ccessiility of television, computer, nd other digitl devices in most fmilies, progrms to promote helthy lifestyle y reducing screen nd video gme time re needed ecuse medi use ws found to shorten sleep time. In cross-sectionl study of 4,511 Portugese children ged 7 9 y sed on questionnires filled out y prents, children who engged in physicl ctivity slept more thn sedentry children [4]. Therefore, physicl ctivity might hve positive effect on sleep durtion. However, sed on ccelerometry, we found negtive ssocition etween sedentry/light physicl ctivities nd sleep durtion (P < 0.01). The negtive ssocition remined significnt when times spent on sedentry nd light physicl ctivities were nlyzed seprtely while controlling for ge, gender, rce/ethnicity, oesity sttus nd center. No ssocition (P = 0.27) ws detected etween sleep durtion nd time spent on MVPA mong our minority children. The lck of ssocition might e ecuse of the low MVPA mong these minority children [16]. To comt childhood oesity, children re encourged to engge in 60 minutes or more per dy of MVPA [27-29]. Therefore, the reltionship etween MVPA nd sleep durtion deserves more ttention. As indicted in the method section, if ccelerometer counts re equl to zero, it is not possile to distinguish quiet time (e.g. lying motionless down quietly to listen to music) from ctul sleep time. Therefore, inclusion of quiet period prior to night sleep would rtificilly inflte the sleep time suggesting tht the true sleep time might ctully e even shorter thn wht we hve reported. Severl met-nlyses documented tht short sleep durtion is ssocited with childhood nd dult oesity [6-9]. A longitudinl study on children lso showed tht shorter sleep durtion t 9 y of ge ws ssocited with n incresed risk of oesity t 12 y of ge [10]. There re limittions to our findings. First of ll, this is cross-sectionl study without control group. Therefore the results cnnot e used to exmine cusl effect of short sleep durtion on oesity risk or to determine whether the underserved minority children were more vulnerle to sleep deprivtion. However, ccelerometry-determined sleep dt on white children, presumly from higher income fmilies, lso showed tht these children were not meeting the sleep recommendtion suggesting tht sleep deprivtion might e universl risk fctor mong ll contemporry US children. Secondly, the sleep dt were collected from children living in southern US metropolitn city nd therefore might not e pplicle to underserved minority children living in other lrge metropolitn cities or suurn nd rurl environments. More studies using ojective mesures of sleep durtion re needed to confirm nd expnd our findings in other regions in the US mong ll children. Conclusions This study presents n ojective ssessment of sleep durtion in lrge smple of underserved minority children living in lrge US metropolitn re. In greement with the findings from the Ntionl Institute of Child Helth nd Humn Development Study of Erly Child Cre nd Youth Development on predominntly white children nd survey y the Ntionl Sleep Foundtion [5,30], underserved lck nd Hispnic children re sleeping less thn 9 h/d on verge. Short sleep durtion hs een ssocited with incresed risk of oesity [5,31,32] nd dverse metolic outcomes [17,33-35]. Lrge-scle longitudinl studies using ojective mesures re needed in order to estlish cuslity etween sleep durtion nd oesity risk nd oesity-relted co-moridities mong children. Arevitions AEE: Activity energy expenditure; BCM: Bylor College of Medicine; CPM: Counts per minute; HPARD: City of Houston Prks nd Recretion Deprtment; MVPA: Moderte-vigorous physicl ctivity; PAR: Physicl ctivity rtio; SES: Socioeconomic sttus. Competing interests The uthors declred tht they hve no competing interests. The contents of this puliction do not necessrily reflect the views or policies of the U.S. Deprtment of Agriculture or mention of trde nmes, commercil products, or orgniztions imply endorsement. Author informtion WWW is the Project Director of the Helthy Kids-Houston project nd Professor of Peditrics t BCM. CLO is the Principl Investigtor of the Helthy Kids-Houston project nd n Administrtor Mnger t HPARD. DL is the Assistnt Director t HPARD. LAM is Co-Investigtor of the Helthy Kids-Houston project nd Senior Superintendent t HPARD. KLK is Project Consultnt of the Helthy Kids-Houston project. ALA is n Electronic Engineer t the USDA/ARS Children s Nutrition Reserch Center. EOS is the Biosttisticin of the Helthy Kids-Houston project nd Professor of Peditrics t BCM. NFB is Professor of Peditrics t BCM nd the Director of the Energy Metolism Lortory t the USDA/ARS Children s Nutrition Reserch Center. Authors contriutions WWW ws responsile for the implementtion of the Helthy Kids-Houston tht generted the sleep dt for the mnuscript. CLO, DL nd LAM identified the community centers to support the project. They lso provided the instructors to help collect the dt. KLK ssisted in suject recruitment. ALA nd NFB ssisted in the plcement of the Acticls on the children nd on converting the Acticl dt into ctivity times nd sleep durtion. EOS ssisted in the study design nd the sttisticl nlysis of the sleep dt. All

7 Wong et l. BMC Pulic Helth 2013, 13:648 Pge 7 of 7 the uthors were involved in the originl study design, ssisted in the implementtion of the project, nd red nd pproved the finl mnuscript. Acknowledgements The study ws funded y the Ntionl Reserch Inititive of the USDA Ntionl Institute of Food nd Agriculture grnt # with support from the USDA/ARS grnt # The study sponsors hd no involvement in the study design; the collection, nlysis, nd interprettion of dt; the writing of the mnuscript; nd the decision to sumit the pper for puliction. Author detils 1 USDA/ARS Children s Nutrition Reserch Center, Deprtment of Peditrics, Bylor College of Medicine, Houston, Texs, USA. 2 Houston Prks nd Recretion Deprtment, Houston, Texs, USA. Received: 23 Jnury 2013 Accepted: 8 July 2013 Pulished: 12 July 2013 References 1. Astrup A: Hve we een rking up the wrong tree: cn good night s sleep mke us slimmer? 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