Enrollment of participants
JECS is a nationwide, government funded birth cohort study begun in 2011 with the aim to elucidate the effects of environmental factors on the health of mother and child [13, 14]. 103,099 pregnancies were enrolled across the fifteen regional centers, which include representation of the diverse social, economic, and urban lifestyles of Japan during a 3-year period ending March 2014. The JECS protocol was approved nationally by the Institutional Review Board (IRB) in compliance with the Ethical Guidelines for Epidemiological Research, published by the Ministry of Education, Culture, Sports, Science and Technology, and the Ministry of Health, Labor and Welfare, and by the Ethics Committees at all participating institutions. Written informed consent was obtained from participants.
Data collection
Two questionnaires per person were completed in the first (mean 16 weeks) and second to third trimesters (mean 27 weeks), addressing past pregnancy history, lifestyle, foods, and sleeping habits including sleep start and waking times. Specifically, sleep questions were administered in the second to third trimesters, and had the following phrasing, “What was the average time to get into bed and turn off the light over the last month?” and “What was your average waking time over the last month?”
Medical records were obtained, including maternal age, parity, height, pre-pregnancy body weight, body weight before delivery, blood glucose during pregnancy (random blood glucose, RBG, see below methods), gestational age at delivery, birth weight, and placental weight. Additional history, including information about Type1 or Type 2 DM or previous GDM diagnosis, and use of steroids during pregnancy were collected. Sleep duration, pre-pregnancy body-mass index (BMI) and gestational weight gain were calculated from raw data.
Glucose tolerance and GDM screening
Glucose tolerance screening and testing for GDM was performed according to the protocols of the Obstetrics Society or Diabetes Society of Japan depending on the local institution using a 2-step protocol (Additional file 1: Figure S1). The first step was screening of random blood plasma glucose levels (RBG) during the first trimester. The second step was screening using either RBG or a fasting 50 g oral glucose challenge test (GCT) in the second trimester. An RBG score of ≧95 mg/dL or a 1 h 50 g GCT result of > 140 mg/dL were scored as a positive screening result. After a positive screening result, a 75 g oral glucose tolerance test (OGTT) was administered with GDM-positive cut-off values of fasting blood plasma glucose of ≤92 mg/dL, 1 h ≥ 180 mg/dL, or 2 h ≥153 mg/dL. In this study, for the purpose of consistency, we selected only subjects with RBG screening data (the majority 82%), to the exclusion of the fasting 50 g GCT-tested subjects (see Statistical analysis, Fig. 1).
Statistical analysis
Participants were excluded based upon missing data (n = 39,991), non-compliant blood glucose screening results (10,949), existing type1 or type 2 diabetes (n = 686), and average sleep duration values less than 3 and over 13 h (n = 2686). Finally, 48,787 subjects were included in the analysis (see Fig. 1).
Sleep duration was categorized according to hours per night (< 5, 5 to < 7, 7 to < 10, and ≥ 10 h), with values estimated from sleep start and waking time survey responses. The 7 to < 10 h group was set as the reference. Because the random blood glucose level data did not have a normal distribution, the median (25th,75th) per sleeping duration were expressed, and the Mann-Whitney test based upon non-Gaussian distribution was used to compare sleep group blood glucose levels. Covariant factors included age, pre-pregnancy BMI, gestational weight gain [15], steroid use during pregnancy, and previous GDM. The adjusted relative risk ratios were calculated using a log-binomial regression model. Statistical analyses were completed using STATA version 11.1 (Texas, USA).