Bedtime Variability and Metabolic Health in Midlife Women: The SWAN Sleep Study.
OBJECTIVE
Circadian misalignment, as seen in shift workers, can disrupt metabolic processes. Associations between sleep timing in nonshift workers and metabolic health are unknown. We examined sleep timing and indices of metabolic health in a community sample of midlife women.


METHODS
Caucasian (n = 161), African American (n = 121) and Chinese (n = 56) non-shift-working women aged 48-58 y who were not taking insulin-related medications, participated in the Study of Women's Health Across the Nation (SWAN) Sleep Study and were subsequently examined approximately 5.39 (standard deviation = 0.71) y later. Daily diary-reported bedtimes were used to calculate four measures of sleep timing: mean bedtime, bedtime variability, bedtime delay and bedtime advance. Body mass index (BMI) and insulin resistance (homeostatic model assessment-insulin resistance, HOMA-IR) were measured at two time points. Linear regressions evaluated whether sleep timing was associated with BMI and HOMA-IR cross-sectionally and prospectively.


RESULTS
In cross-sectional models, greater variability in bedtime and greater bedtime delay were associated with higher HOMA-IR (β = 0.128; P = 0.007, and β = 0.110; P = 0.013, respectively) and greater bedtime advance was associated with higher BMI (β = 0.095; P = 0.047). Prospectively, greater bedtime delay predicted increased HOMA-IR at Time 2 (β = 0.152; P = 0.003). Results were partially explained by shifted sleep timing on weekends.


CONCLUSIONS
Frequent shifts in sleep timing may be related to metabolic health among non-shift working midlife women.


CONCLUSIONS
A commentary on this article appears in this issue on page 269.
Department of Psychology, University of Pittsburgh, Pittsburgh, PA.
Briana J
BJ
Taylor
Department of Psychiatry, University of Pittsburgh School of Medicine, Pittsburgh, PA.
Karen A
KA
Matthews
Department of Psychiatry, University of Pittsburgh School of Medicine, Pittsburgh, PA.
Brant P
BP
Hasler
Department of Psychology, University of Pittsburgh, Pittsburgh, PA.
Kathryn A
KA
Roecklein
Department of Psychiatry, University of Pittsburgh School of Medicine, Pittsburgh, PA.
Christopher E
CE
Kline
Department of Psychiatry, University of Pittsburgh School of Medicine, Pittsburgh, PA.
Daniel J
DJ
Buysse
Departments of Psychiatry and Preventive Medicine, Rush University, Chicago, IL.
Howard M
HM
Kravitz
Department of Psychology, University of Pittsburgh, Pittsburgh, PA.
Alaina G
AG
Tiani
Department of Epidemiology, University of Michigan, Ann Arbor, MI.
Sioban D
SD
Harlow
Department of Psychiatry, University of Pittsburgh School of Medicine, Pittsburgh, PA.
Martica H
MH
Hall
D001741:Black or African American / Q000706:statistics & numerical data; D044466:Asian People / Q000706:statistics & numerical data; D015992:Body Mass Index; D003430:Cross-Sectional Studies; D004734:Energy Metabolism; D005260:Female; D006306:Health Surveys; D006801:Humans; D007333:Insulin Resistance / Q000502:physiology*; D008875:Middle Aged; D017286:Polysomnography; D012890:Sleep / Q000502:physiology*; D013997:Time Factors; D044465:White People / Q000706:statistics & numerical data; D016387:Women's Health / Q000706:statistics & numerical data*
circadian; insulin resistance; sleep