Sleep is a crucial aspect of recovery and overall well-being, yet many adolescent female athletes experience disruptions influenced by hormonal changes throughout the menstrual cycle. While research on elite adult athletes has explored sleep disturbances related to training and competition, there is limited understanding of how menstrual cycle fluctuations impact sleep in younger athletes. Variations in estrogen and progesterone levels across the follicular and luteal phases are known to affect sleep patterns, including sleep quality, duration and onset latency. Increased progesterone in the luteal phase, for example, has been linked to poorer sleep efficiency and greater difficulty falling asleep. Additionally, menstrual-related symptoms such as cramps, fatigue, and mood changes may further interfere with rest and recovery. Given the importance of sleep for athletic performance and well-being, the first chapter of this thesis seeks to examine the current knowledge on how different phases of the menstrual cycle influence sleep in adolescent female athletes. The second chapter of this thesis presents an original study that examined how different phases of the menstrual cycle affect sleep in adolescent female athletes. Over the course of three menstrual cycles, ten participants (14.5 ± 1.9) who were in their early-to-mid and mid-to-late pubertal stages, tracked their sleep using both subjective and objective methods. They recorded sleep duration and perceived sleep quality in a sleep diary. Simultaneously, wrist-worn actigraphy devices (Fitbit) measured objective sleep metrics, including sleep duration, sleep quality, sleep onset latency, wake episodes, and wake time. The reliability of both subjective and objective sleep measures in adolescent female athletes during the follicular phase (FP) and luteal phases (LP) of the menstrual cycle was examined. Subjective measures demonstrated poor reliability in the luteal phase (sleep duration: ICC = -0.22; sleep quality: ICC = -0.49) but fair to good reliability during the follicular phase for sleep duration (ICC= 0.69; CV = 4.9%) and sleep quality (ICC = 0.57; CV = 8.9%). Objective measures showed poor reliability during the luteal phase for sleep duration (ICC = -0.17; CV 11%) and sleep quality (ICC= 0.00; CV = 9.2%) but fair reliability during the follicular phase for sleep duration (ICC = 0.38; CV = 7.2%) and sleep quality (ICC = 0.51; CV = 5.8%). When comparing objective data to subjective data, participants overestimated subjective sleep duration compared to the objective sleep duration data with the average difference being (1.38 h; p = 3.00 x 10-47; effect size = 1.20). When comparing the two menstrual phases, sleep onset latency was non-significantly worse during the luteal phase compared to the follicular phase (LP = 20.8 mins; FP = 16.0 mins; p= 0.68; effect size = -1.54). Objective sleep quality was non-significantly worse in the luteal phase in comparison to the follicular phase (LP = 75.4; FP = 77.1%; p = 0.34; effect size = 1.56) and sleep duration was non-significantly shorter in the luteal phase compared to the follicular phase (LP = 9.04 hrs; FP = 9.09 hrs; p = 0.72; effect size = 0.24). Lastly, the mid-late pubertal group had 3 significantly longer sleep duration during the follicular phase compared to the early-mid pubertal group (7.98 vs 7.44 h; p =0.03; effect size = -4.36 ± 0.12). The final chapter discusses the study’s findings, indicating that although both subjective and objective measures have their advantages, the subjective measures tended to be more reliable during specific menstrual cycle phases. The participants underestimated their sleep duration, reinforcing the importance of combining objective and subjective measures to get more accurate assessments of sleep duration. The adolescent female athlete’s sleep quality and sleep onset latency can slightly be impacted by the menstrual cycle phases and sleep patterns were subtly worse during the luteal phase in comparison to the follicular phase; however, there were no statistically significant differences observed in the sleep measures studied. Sleep patterns do seem to improve in the mid-late pubertal maturation stage when compared to the early-mid pubertal stage. The data show that it is important to understand and take into consideration the menstrual cycle phases and the maturation stages of our athletes when creating training and recovery plans for these adolescent female athletes; however, more investigation is required to understand the potential effects of the menstrual cycle on sleep disturbances and its effect on female participation and performance in sport.