Abstract Introduction Menstrual cycle regularity is an important marker of women’s health. Abnormalities are associated with serious health complaints, e.g., infertility, cardiovascular, and metabolic disorders. Cycle irregularity is also linked to depression, anxiety and poor quality sleep. In fact, poor sleep and circadian misalignment may precede menstrual irregularity for some. This study describes sleep and affective characteristics of women with menstrual cycle irregularity compared to regularly cycling women and examined the individual contributions of sleep and menstrual regularity to affective symptoms. Methods N=314 (Mage=20.95, SD=2.35) women provided reports of menstrual health characteristics (frequency, duration, related symptoms), sleep over the past month (Pittsburgh Sleep Quality Index; PSQI) and on the previous night, as well as mood (Center for Epidemiologic Studies Depression Scale) and anxiety (State Trait Anxiety Inventory) symptoms. Results Among the 20.4% (n=64) of women who endorsed “always irregular” periods, mean cycle length (m=31.33 days, sd=8.5) was significantly greater than among women more regular periods (m= 27.93, sd=3.83, t(55.37)=-2.78, p=.007). This subgroup also reported higher PSQI scores (t(181)=-2.56, p=.011), longer SOL (t(282)=-3.00, p=.003), poorer sleep quality overall (t(299)=-2.35, p=.02), and poorer sleep quality on the previous night (t(300)=2.70, p=.007). Irregular cyclers reported significantly more depressive (t(284)=-2.18, p=.03) but not anxiety symptoms. When sleep and menstrual irregularity were entered into a hierarchical linear regression to examine their relative influence on depressive symptoms (F[2,174]=13.15, p<.001, R2=.13), sleep remained the only significant predictor (b= 1.13, p <.001) and menstrual cycle effects were no longer significant. Conclusion In line with previous studies, cycle irregularity was associated with poorer sleep quality and depressive symptoms. Sleep quality was found to be a more robust predictor of depression than menstrual cycle irregularity, suggesting that sleep may underlie the affective disturbances in women with more irregular menstrual cycles. Support n/a