Relationship Between Obesity and Exposure to Light at Night
Relationship Between Obesity and Exposure to Light at Night
The subjects were participants in the Breakthrough Generations Study, a cohort study of over 113,000 women aged 16 years or older who were living in the United Kingdom and were recruited during 2003–2012. The cohort has been described in detail elsewhere. Initial recruits were registered supporters of the Breakthrough Breast Cancer charity (3.8%) or women who referred themselves to the study (22.9%). Participants were able to nominate their friends and family to join the study, and the majority of participants were recruited using this method. Ethics approval for the study was obtained from the South Thames Multicentre Research Ethics Committee. Participants gave informed consent.
The participants completed a detailed baseline postal questionnaire that included a comprehensive assessment of breast cancer risk factors, including weight, height, and waist and hip circumferences, and whether these factors were measured on the day of the questionnaire, had been measured recently, or were estimated. BMI, waist:hip ratio (WHR), and waist:height ratio (WHtR) were calculated from these measurements. Given that BMI cannot distinguish between persons with high muscle mass and persons with excess adipose tissue, we also examined WHR, WHtR, and waist circumference; these measures are considered better markers of abdominal obesity, which is thought to be a key risk factor for disease. Four BMI groups were defined: underweight (<18.5), normal-weight (18.5–<25), overweight (25–<30), and obese (≥30). Four WHR groups were defined: underweight (<0.67), normal-weight (0.67–<0.80), overweight (0.80–<0.85), and obese (≥0.85). Two WHtR groups were defined, depending on age: low (<0.5 if aged <40 years, <0.55 if aged <50 years, and <0.6 if aged ≥50 years) and high (≥0.5 if aged <40 years, ≥0.55 if aged <50 years, and ≥0.6 if aged ≥50 years). Waist circumference was dichotomized as low (<0.88 m; <35 inches) or high (≥0.88 m; ≥35 inches).
Exposure to LAN was assessed through participants' answers to a categorical-response question about the lightness of the room they slept in; the response categories were "light enough to read"; "light enough to see across the room, but not read"; "light enough to see your hand in front of you, but not to see across the room"; and "too dark to see your hand, or you wear a mask." The 2 lightest categories were combined because of small numbers in the lightest group (<1%). The questionnaire also asked participants to specify what times they usually went to sleep and woke up on weekdays. Answers were used to derive both the average number of hours of sleep per night and a dichotomous "nonpeak sleep" variable, where "nonpeak sleep" was defined as going to sleep at or after 2:00 AM or rising for the day at or before 1:00 AM and "usual sleep" was defined as all other sleeping patterns.
Information on socioeconomic status was obtained by deriving the participant's ACORN (A Classification of Residential Neighborhoods) score, an indicator of socioeconomic status based on residential postcodes (http://acorn.caci.co.uk). Lifestyle factors entered into the multivariate models included alcohol consumption (units/week; 1 unit = 8 g), current smoking, average number of hours of sleep per night, strenuous physical activity (hours/week), recent participation in night-shift work, and having a child under 5 years of age.
Participants were eligible for these analyses if they reported anthropometric details and information about LAN exposure or sleeping patterns in the recruitment questionnaire. Multinomial logistic regression was used to examine the relationship between obesity measures and both LAN and sleeping patterns separately, after adjustment for age and the covariates described. All reported P values were 2-sided. All statistical analyses were performed in Stata, version 10.1 (StataCorp LP, College Station, Texas).
Methods
Participants
The subjects were participants in the Breakthrough Generations Study, a cohort study of over 113,000 women aged 16 years or older who were living in the United Kingdom and were recruited during 2003–2012. The cohort has been described in detail elsewhere. Initial recruits were registered supporters of the Breakthrough Breast Cancer charity (3.8%) or women who referred themselves to the study (22.9%). Participants were able to nominate their friends and family to join the study, and the majority of participants were recruited using this method. Ethics approval for the study was obtained from the South Thames Multicentre Research Ethics Committee. Participants gave informed consent.
Variables
The participants completed a detailed baseline postal questionnaire that included a comprehensive assessment of breast cancer risk factors, including weight, height, and waist and hip circumferences, and whether these factors were measured on the day of the questionnaire, had been measured recently, or were estimated. BMI, waist:hip ratio (WHR), and waist:height ratio (WHtR) were calculated from these measurements. Given that BMI cannot distinguish between persons with high muscle mass and persons with excess adipose tissue, we also examined WHR, WHtR, and waist circumference; these measures are considered better markers of abdominal obesity, which is thought to be a key risk factor for disease. Four BMI groups were defined: underweight (<18.5), normal-weight (18.5–<25), overweight (25–<30), and obese (≥30). Four WHR groups were defined: underweight (<0.67), normal-weight (0.67–<0.80), overweight (0.80–<0.85), and obese (≥0.85). Two WHtR groups were defined, depending on age: low (<0.5 if aged <40 years, <0.55 if aged <50 years, and <0.6 if aged ≥50 years) and high (≥0.5 if aged <40 years, ≥0.55 if aged <50 years, and ≥0.6 if aged ≥50 years). Waist circumference was dichotomized as low (<0.88 m; <35 inches) or high (≥0.88 m; ≥35 inches).
Exposure to LAN was assessed through participants' answers to a categorical-response question about the lightness of the room they slept in; the response categories were "light enough to read"; "light enough to see across the room, but not read"; "light enough to see your hand in front of you, but not to see across the room"; and "too dark to see your hand, or you wear a mask." The 2 lightest categories were combined because of small numbers in the lightest group (<1%). The questionnaire also asked participants to specify what times they usually went to sleep and woke up on weekdays. Answers were used to derive both the average number of hours of sleep per night and a dichotomous "nonpeak sleep" variable, where "nonpeak sleep" was defined as going to sleep at or after 2:00 AM or rising for the day at or before 1:00 AM and "usual sleep" was defined as all other sleeping patterns.
Information on socioeconomic status was obtained by deriving the participant's ACORN (A Classification of Residential Neighborhoods) score, an indicator of socioeconomic status based on residential postcodes (http://acorn.caci.co.uk). Lifestyle factors entered into the multivariate models included alcohol consumption (units/week; 1 unit = 8 g), current smoking, average number of hours of sleep per night, strenuous physical activity (hours/week), recent participation in night-shift work, and having a child under 5 years of age.
Statistical Analyses
Participants were eligible for these analyses if they reported anthropometric details and information about LAN exposure or sleeping patterns in the recruitment questionnaire. Multinomial logistic regression was used to examine the relationship between obesity measures and both LAN and sleeping patterns separately, after adjustment for age and the covariates described. All reported P values were 2-sided. All statistical analyses were performed in Stata, version 10.1 (StataCorp LP, College Station, Texas).
