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Sixty-seven percent of women included in the sleep study were classified as premenopausal (no change in menstrual bleeding regularity) and early perimenopasual (menses in the preceding three months with an increase in bleeding irregularity), 21 percent were identified as late perimenopasual (menses in the previous 12 months, but not in the previous three months), and 11 percent were classified as postmenopausal (12 months of amenorrhea). The median age was 52 years and median body mass index (BMI) was 28 kg. Women slept for a median of nearly six-and-a-half hours per night of the study with a median 40 minutes of wakefulness following sleep onset.

The cross-sectional sleep substudy utilized information obtained from the Study of Women's Health Across the Nation (SWAN) in addition to data collected for a sleep-study profile from 365 Caucasian, African-American and Chinese women. Data were gathered through objective in-home polysomnography and subjective measures such as the Pittsburgh Sleep Quality Index.

SWAN is a multiethnic, community-based, multisite cohort study of the menopausal transition. The study included a comprehensive analysis of sleep (a single sleep study that lasted for three nights) nested within an ongoing longitudinal study and was conducted at four of the seven SWAN clinical sites during 2003-2005.

The sleep study for the sleep profile began within seven days of the beginning of the follicular phase of the menstrual cycle in premenopausal women who were still menstruating. Polysomnography was conducted in the homes of participants.

Women using hormone therapy were excluded from the study, because it would confound the hormone results. Some studies provide evidence that HT can improve some, but n
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