menstrual cycle effects in females. Humanlike sex differences in sleep spindle activity and sleep-dependent learning were observed in the dog. A direct link to specific sexual hormones could not be established, but the effects were stronger in intact animals. Women tend to have 0.16 more sleep spindles per minute than men ( ⁠i.e. roughly 9–⁠10 more over an hour's time). A female advantage has been found for episodic, emotional, and spatial memories as well as recognition of odours, faces, and pictures. These differences are believed to be due to hormonal influence, especially that of estrogen. The female sex hormone estrogen primarily influences sexual maturation and reproduction, but has also been found to facilitate other brain functions, including cognition and memory. On verbal tasks where women scored higher than men, women scored higher during the mid-luteal phase, when women have higher estrogen levels, when compared to the menstrual phase. A recent study found that local brain estrogen production within cognitive circuits may be important for the acquisition and consolidation of memories. Recent experiments concerning the relationship between estrogen and the process of offline memory consolidation have also focused on sleep spindles. Genzel and colleagues determined that there was a menstrual effect on declarative and motor performance, meaning that women in the mid-luteal phase (high estrogen) performed higher than the other female participants. Women in the luteal phase were also the only participants to experience an increase in spindles after learning, which led to the conclusion that the effect of the menstrual cycle may be mediated by spindles and female hormones. References Sleep physiology Electroencephalography Sleep Neural circuits