Sleep Duration as an Indirect Link Between Sleep Timing and Weight in Midlife Women

Midlife women are at risk of obesity. Poor sleep outcomes including inadequate sleep duration and variable sleep timing are risk factors for obesity, but there is a lack of understanding on how specific sleep constructs uniquely or concurrently are associated with weight outcomes in this population. This study examined the unique association of sleep timing with weight outcomes and how sleep timing works in conjunction with sleep duration to influence weight in midlife women. An archival analysis was performed using the Midlife in the United States-II study (MIDUS-II). The sample consisted of 132 midlife women (40-64; M = 52.9, standard deviation = 6.94). Sleep timing (mean sleep time, variability) and duration were measured through actigraphy and daily sleep diaries. Weight was assessed using body mass index (BMI) and waist circumference measures. PROCESS mediation and moderation analyses assessed direct, indirect, and moderating pathways. Sleep duration emerged as an indirect link between sleep timing (mean and variability) and weight outcomes (95% CI = 0.0001-0.0123; 95% CI = 0.0007-0.0378; 95% CI = 0.0079-0.1006). Sleep timing (95% CI = -0.0144 to 0.0076; 95% CI = -0.0358 to 0.0219) and sleep time variability (95% CI = -0.0124 to 0.0438; 95% CI = -0.0533 to 0.0939) were not directly associated with BMI or waist circumference. Also, sleep timing and duration did not interact to influence weight outcomes. Sleep duration, rather than sleep timing, is associated with weight outcomes, and is an indirect link in the sleep timing and weight outcomes association. Future work is needed to further disentangle the impact of sleep on weight in midlife women using prospective studies, implementing daily assessments of sleep behavior.