1. In this study, women with regular periods and in the luteal phase of their cycle significantly improved their memory performance overnight in comparison to women taking oral contraceptives and in the follicular phase of their cycle.
2. Women with higher levels of progesterone or progestin had more speed spindles during non-rapid eye-movement stage 2 (NREM2) sleep in comparison to naturally cycling women in their follicular phase, with lower levels of progesterone.
Evidence Rating Level: 2 (Good)
Previous studies suggest sex hormones, such as progesterone, may have a role in memory consolidation during sleep. In particular, changes in sleep spindle activity during non-rapid eye-movement stage 2 (NREM2) sleep have been associated with memory consolidation. Levels of progesterone naturally fluctuate during the menstrual cycle; they are elevated during the luteal phase and decreased in the follicular phase. In contrast, women on oral contraceptives (OC) have low levels of endogenous progesterone but have elevated potent synthetic progestin. The objective of this study was to investigate the effect of progesterone levels in three groups of women (natural cycling women in the luteal phase; naturally cycling women in the follicular phase; women using OCs) on sleep spindle density (during NREM2) and declarative memory consolidation.
This study included women with a regular menstrual cycle or who used OCs. Exclusion criteria included: travelling over 3 time zones within the last 3 months, current or past drug abuse, coffee consumption exceeding 3 units/ days, and diagnosis of a neurological or endocrine disorder. Women participated in a 7-day study period during which study outcomes assessed salivary progesterone levels and sleep spindle density (via polysomnography), as well as declarative memory performance (via a word pair association task pre and post experimental night).
Results showed memory improvement and an increase in density of fast spindles in women in their luteal phase and women using OC, compared to women in the follicular phase. Progesterone levels were highest in naturally cycling women who were in their luteal phase. However, this study was limited by lack of measurement of synthetic progestin levels in women using OCs, and lack of control for dosing of synthetic hormones in the OC group. Nonetheless, this study suggests that cyclical hormone fluctuations may affect memory performance, which could have clinical implications in the future.
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