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Sleep is crucial for optimal well-being, and sex differences in sleep quality have significant implications for women’s health. We review the current literature on sex differences in sleep, such as differences in objective and subjective sleep measures and their relationship with aging. We then discuss the convincing evidence for the role of ovarian hormones in regulating female sleep, and survey how these hormones act on a multitude of brain regions and neurochemicals to impact sleep. Lastly, we identify several important areas in need of future research to narrow the knowledge gap and improve the health of women and other understudied populations.

Gonadal hormones contribute to the sexual differentiation of brain and behavior throughout the lifespan, from initial neural patterning to “activation” of adult circuits. Sexual behavior is an ideal system in which to investigate the mechanisms underlying hormonal activation of neural circuits. Sexual behavior is a hormonally regulated, innate social behavior found across species. Although both sexes seek out and engage in sexual behavior, the specific actions involved in mating are sexually dimorphic. Thus, the neural circuits mediating sexual motivation and behavior in males and females are overlapping yet distinct. Furthermore, sexual behavior is strongly dependent on circulating gonadal hormones in both sexes. There has been significant recent progress on elucidating how gonadal hormones modulate physiological properties within sexual behavior circuits with consequences for behavior. Therefore, in this mini-review we review the neural circuits of male and female sexual motivation and behavior, from initial sensory detection of pheromones to the extended amygdala and on to medial hypothalamic nuclei and reward systems. We also discuss how gonadal hormones impact the physiology and functioning of each node within these circuits. By better understanding the myriad of ways in which gonadal hormones impact sexual behavior circuits, we can gain a richer and more complete appreciation for the neural substrates of complex behavior.

Aggressive interactions lead to changes in both future behavior and circulating testosterone (T) concentrations in animals across taxa. The specific neural circuitry and neurochemical systems by which these encounters alter neuroendocrine functioning are not well understood. Neurons expressing the inhibitory and stimulatory neuropeptides, RFamide-related peptide (RFRP) and kisspeptin, respectively, project to neural loci regulating aggression in addition to neuroendocrine cells controlling sex steroid production. Given these connections to both the reproductive axis and aggression circuitry, RFRP and kisspeptin are in unique positions to mediate post-encounter changes in both T and behavior. The present study examined the activational state of RFRP and kisspeptin neurons of male C57BL/6 mice following an aggressive encounter. Both winners and losers exhibited reduced RFRP/FOS co-localization relative to handling stress controls. Social exposure controls did not display reduced RFRP neuronal activation, indicating that this effect is due to aggressive interaction specifically rather than social interaction generally. RFRP neuronal activation positively correlated with latencies to display several offensive behaviors within winners. These effects were not observed in the anteroventral periventricular (AVPV) nucleus kisspeptin cell population. Together, these findings point to potential neuromodulatory role for RFRP in aggressive behavior and in disinhibiting the reproductive axis to facilitate an increase in T in response to social challenge.

Twenty years ago, the hypocretins/orexins were independently identified by de Lecea et al. and Sakurai et al. as novel neuropeptides expressed exclusively in the lateral hypothalamus (de Lecea et al., 1998; Sakurai et al., 1998). Although neither paper mentioned sleep, an explosion of research over the intervening twenty years has revealed a critical role for Hcrt in promoting arousal and regulating sleep/wake stability. Moreover, the Hcrt system has proven especially amenable to genetically and anatomically targeted manipulations, facilitating rapid and systematic description of the circuit mechanisms mediating arousal. This chapter will review the evidence revealing Hcrt's role in arousal and the insights gained by using the Hcrt system as a foothold to gain access to arousal circuits.

At the turn of the millennium, a neuropeptide with pronounced inhibitory actions on avian pituitary gonadotrophin secretion was identified and named gonadotrophin-inhibitory hormone (GnIH). Across bird species, GnIH acts at the level of the pituitary and the gonadotrophin-releasing hormone (GnRH) neuronal system to inhibit reproduction. Subsequent to this initial discovery, orthologues of GnIH have been identified and characterised across a broad range of species. In many vertebrates, the actions of GnIH and its orthologues serve functional roles analogous to those seen in birds. In other cases, GnIH and its orthologues exhibit more diverse actions dependent on sex, species, season and reproductive condition. The present review highlights the discovery and functional implications of GnIH across species, focusing on research domains in which the significance of this neuropeptide has been explored most.

Throughout most of the ovulatory cycle, estrogen negative feedback restrains the GnRH neuronal system. Just before ovulation, however, estrogen negative feedback is removed to permit stimulation of the preovulatory GnRH/LH surge (positive feedback) by the circadian clock in the suprachiasmatic nucleus (SCN). The mammalian ortholog of avian gonadotropin-inhibitory hormone, RFamide-related peptide 3 (RFRP-3), participates in the circadian-timed removal of estrogen negative feedback to permit the LH surge. The present study examined the specific neurochemical means by which the SCN controls RFRP-3 activity and explored whether the RFRP-3 system exhibits time-dependent responsiveness to SCN signaling to precisely time the LH surge. We found that RFRP-3 cells in female Syrian hamsters (Mesocricetus auratus) receive close appositions from SCN-derived vasopressin-ergic and vasoactive intestinal peptide (VIP)-ergic terminal fibers. Central VIP administration markedly suppressed RFRP-3 cellular activity in the evening, but not the morning, relative to saline controls, whereas vasopressin was without effect at either time point. Double-label in situ hybridization for Rfrp-3 and the VIP receptors VPAC1 and VPAC2 revealed that the majority of RFRP-3 cells do not coexpress either receptor in Syrian hamsters or mice, suggesting that SCN VIP-ergic signaling inhibits RFRP-3 cells indirectly. The timing of this VIP-mediated disinhibition is further coordinated via temporally gated responsiveness of RFRP-3 cells to circadian signaling. Together, these findings reveal a novel circadian hierarchy of control coordinating the preovulatory LH surge and ovulation.