Nathaniel Kleitman and his colleagues (1937) [11] were the first to study human sleep, and their sample of 35 young adults included 11 women, primarily graduate students at the University of Chicago. They described each adult’s age, sex, history of smoking and alcohol consumption, and sleep habits, but no mention was made of women’s menstrual cycles. In 1966, Williams, Agnew, and Webb published the first report on polysomnography (PSG) sleep characteristics in women. In that study of 16 women, researchers made a point of not monitoring PSG during days of menstrual flow, but exact phase of their cycles were not reported. During menstrual flow, there may be uterine cramping and pain, but all gonadal hormones would be secreted at basal levels, and comparable to levels in men of the same age [12].

A few years later, Williams and colleagues (1974) [13] compiled PSG data in a book format organized by sex and decade of life. These researchers were the first to control for potential menstrual cycle effects by studying women of childbearing age only during their follicular phase. In the follicular phase, before ovulation, progesterone levels remain low. Sample sizes were small (ten women between 20 and 29 years old and ten women between 30 and 39 years old) and included only healthy females across the lifespan. The small samples of women between 40 and 49 years old and between 50 and 59 years old were likely in various stages of menopausal transition [13]. It was in these comparison studies that older women appeared to have more delta sleep (stages 3 and 4) than men of the same age [13].

Ernest Hartmann (1966) [14] was the first sleep researcher to study women’s sleep architecture as it changed during the menstrual cycle. As a psychiatrist, he saw mood disorders in female patients that appeared to fluctuate in severity by phase of the menstrual cycle. He focused on REM sleep and reported PSG findings from four young healthy women and three psychiatric inpatients. Six of these seven women complained of premenstrual symptoms and the seventh was taking hormone therapy and hence not ovulating. He noted differences in rapid eye movement (REM) sleep when the follicular (preovulation) phase was compared to the luteal (postovulation) phase. However, findings from this type of small sample were not particularly generalizable to the population of all young women, and his findings have not been replicated. In the next published study on women’s sleep, Ho (1972) [15] reported no changes in REM across three menstrual cycles in her sample of six healthy women. Three of the women in Ho’s study were taking oral contraceptives and had less delta sleep (stages 3 and 4) while the other three women had an increase in delta sleep from their follicular to luteal phase. Ho’s (1972) [15] report was published as an abstract and presented at a national sleep conference. More complete details of her dissertation research were also published [16].