Sleep regularity and body mass index: findings from a prospective study of first-year college students

Using data from a large, prospective study of sleep in first-year college students, we examined whether students' sleep regularity is associated with body mass index (BMI) and BMI change (∆BMI) during their first college semester. In a subset of participants, we also tested whether dim light melatonin onset (DLMO) phase and DLMO-bedtime phase angle are associated with BMI and ∆BMI.

Analyses included data from 581 students (mean age = 18.7 ± 0.5 years; 58% female; 48% non-white) who had their height and weight assessed at the start of classes (T1) and end of 9 weeks. Participants completed online daily sleep diaries from which total sleep time (TST) and the sleep regularity index (SRI) were calculated. Among participants who completed a DLMO protocol (n = 161), circadian phase was quantified by DLMO and circadian alignment by DLMO-bedtime phase angle. Data were analyzed with linear regressions that controlled for sex and average TST.

Average SRI was 74.1 ± 8.7 (range: 25.7; 91.6). Average BMI at T1 was 22.0 ± 3.5 and participants gained 1.8 ± 2.4 kg (range: -7.2; 11.4); 39% gained 2-5 kg, 8% gained >5 kg. Lower SRI was associated with greater BMI at T1 (B = -0.06 [95% CI: -0.09; -0.02], p = 0.001) but not with ∆BMI (p = 0.062). Average TST was not significantly associated with BMI or ∆BMI, nor were circadian phase and alignment in the subsample (p's > 0.05).

Sleep regularity is an understudied but relevant sleep dimension associated with BMI during young adulthood. Our findings warrant future work to examine longer-term associations between sleep regularity and weight gain.