The current study is one of several studies based on a data collection in a Norwegian top series team in women's football (27). The team was selected and invited to participate in the current study based on their cooperation with the Olympic Sport center in middle Norway. The Olympic Sport center works with elite coaches, elite athletes and elite teams, to help them become international elite athletes. The coaching staff were first informed about the research project, where the aim of the study was explained and discussed in detail. The coaching staff agreed to participate based on their belief that the intervention might affect the recovery of the players. The team of 25 players were then informed about the project and the players who agreed to participate were instructed to sign a consent form approved by the local Regional Committee for Medical and Health Research Ethics (REC) in Central Norway (project ID 2017/2072/REK midt). Out of the team of 25 players, twenty-one football players (mean age 23.68 ± 2.8, range 20–29 years) signed the consent form and agreed to participate in the current study.

Once the players were enrolled in the study the players were instructed about the data collection process and timeline. For the current study data were collected by day-to-day reports from the players of their subjective sleep. The data collection of day-to-day reports was reported by a smart phone application, PM Reporter Pro sport logging system (58), and the collection started on the 1st of February and lasted until the 31st of October the same year. The current study was designed as a quasi-experiment, where each player was their own control based on a control period compared with an intervention period. The first period of the study, from the 1st of February until the 30th of April, was the control period (Exp1) where the players completed their normal routines with trainings, games, and normsl recovery, but no use of BEMER therapy. The period from 1st of May until 31st of May was the period where the players were introduced for the intervention and got familiarized with the BEMER therapy device (Exp2). The intervention included BEMER therapy each morning and evening, and after high intensity trainings and games. The period from 1st of June until the 31st of October was the experiment period. However, because of extraordinary loads due to qualifying games for the Champions League and play offs in the Norwegian national cup in October, this period was divided into two periods; from 1st of June until the 30th of September (Exp3) where players completed training and games that were comparable with the control period (Exp1), and from the 1st until the 31st of October (Exp4) where players were exposed to extraordinary loads that were not comparable with the control period.

The day-to-day data collection of subjective sleep was collected by using a smartphone application, PM Reporter Pro, which is a part of the PMSys online sports logging system (58). The application scheduled for push messages to remind the participants to report data every morning on a daily basis.

Demographics and descriptive statistical analyses were conducted initially with IBM SPSS (version 27.0) and results were presented as means and ± standard deviations (S.D.). The days/nights in the data collection period were classified as training day/night (TN) if the players completed an ordinary training session that day, and that the day/night was not close to a football game. Days/nights that were close to a football game were classified as night before game (NBG) if the night was the night before a football game, game night (GN) if the night was the night following the day of the football game, game night plus 1 (GN1) if the night was the night one night following game night, and game night plus 2 (GN2) if the night was two nights following the game night. Each date of the night associates to the date of the day where the need for recovery and sleep are developed. Thus, the date of training or game day equals the date of sleep night, regardless of if sleep starts before or after midnight. Dates of days/nights without observations were omitted for further analyses.

The players' means of general training days/nights (TN) were calculated as their baseline values in each period of the experiment (Exp1, Exp2, Exp3 and Exp4), and their values on nights close to football games (NBG, GN, GN1 and GN2) were used to calculate possible differences to their baseline values in each period of the experiment. The period in the experiment where the players experienced extraordinary loads that were not comparable with the control period (Exp4), there were occasions of successive games with only one day between and significant data loss. Data from this period was therefore omitted from further analyses.

The different classified types of nights were classified within each of the periods in the experiment (Exp1, Exp2, Exp3 and Exp4). Accordingly, the players in the experiment completed BEMER therapy treatment in various degree, and the real time each player completed therapy treatment was collected from their BEMER Box Professional control units at the end of the experiment (after 31st of October). The actual time the players were exposed to BEMER therapy treatment was defined as BEMER Time (BMT). For further analyses the types of nights were categorized as the variable Type, the different periods in the experiment, control period (Exp1) and experiment periods (Exp2, Exp3 and Exp4) were categorized as the variable Exp.

The subjective sleep data (SSH and SLQ) and the category variable Exp was analyzed as three-ways factorial design by univariate analysis of variance ANOVA with False Discovery Rate adjusted p-values (59) and by multivariate analysis of variance using FFMANOVA (60, 61). The two category variables and the two-ways interactions between them were used as design variables in the analyses. The analysis was performed in R version 4.2.1 using the FFMANOVA R package (https://cran.r-project.org/package=ffmanova) (62).

The results in the current study found that subjective measured sleep duration and sleep quality decreases significantly on game nights in both the control- and the experiment period (Table 2 and Figure 1). The significant decrease in sleep status on game nights paradoxically occurs in a situation where the elite women's football players need effective recovery caused by the extraordinary loads from the game. The decrease in the players' sleep status when their recovery needs are raised, represents a sleep disturbance (33, 34). A possible explanation is that the sleep disturbance is caused by the extraordinary physical and psychological loads on game days, causing emotional arousals and physical symptoms related to inflammation processes caused by muscle ruptures (21, 29, 33, 34, 38, 63). Interestingly, the players' sleep status on days following football games is significantly below their baseline sleep status for at least 3 days after game days in the control period (Table 2 and Figure 1). Interestingly, the recovery time for football players to achieve metabolic homeostasis after football games is found to be until 72–96 h (31, 64). Thus, the results in the current study show that the players' sleep status is significantly below their baseline sleep status on nights related to football games before the introduction of BEMER therapy. Therefore, it is reason to believe that the players in the current study are not fully recovered in the days after football games in the control period (Table 2 and Figure 1).

After the introduction of BEMER therapy the players subjective sleep duration and sleep quality were significantly increased compared to the control period on game nights and the following two nights (Table 2 and Figure 1). Both the players' subjective sleep duration and sleep quality were above the players' baseline sleep status on general training days on both game nights and the following two nights. The results in the current study also show that the players subjective sleep duration improves every night in the nights following game nights after the introduction of BEMER therapy.

Accordingly, after the introduction of BEMER therapy the players' subjective sleep quality does not drop below their baseline sleep status on game nights and increases significantly on the second night after game night (Figure 1). Before the introduction of BEMER therapy the players sleep quality increased the two days following games, but from a score significantly below their baseline sleep status on general training days (Figure 1). Thus, it might be because the players needed to catch up their sleep on days following game nights to become fully recovered.

A potential explanation of the current results might be that the players' recovery has improved in the experiment period compared to the control period. BEMER therapy treatment is found to improve vasomotion (45, 46, 49), and when blood perfusion improves it results in improved performance recovery (50). The recovery improves because of increased nutrition and oxygen delivery to cells, and transportation of waste and carbon dioxide from metabolism away from the cells (53). The possible effect on the players' vasomotion might be the reason why BEMER influences the players ability to gain more normal sleep after extraordinary loads such as football games (27).

In contrast to other studies that detected no change or a decrease in athletes' sleep status pre-competitions (65), the results in the current study show that the elite women's football players' sleep status on the nights before football games are significantly above their baseline sleep status (Table 2 and Figure 1). The current results are valid for both the control- and experiment period. The results indicate that the players are aware of their need to gain optimal sleep before football games and take actions to be fully recovered and ready for the game.

The playerś subjective experiences of possible experienced effects from the BEMER therapy treatment at the end of the intervention also show that the players are well above average satisfied with their experienced effects on their restitution (SRC), their sleep (SSL), and on their muscle soreness (SMC).

The results in the current study are in line with the results in a recent study that investigated effects from BEMER therapy on objective detected sleep, which found that total sleep time and most sleep stages significantly increased (27). Thus, the results in the current study show that BEMER therapy improves the elite women's football players subjective sleep duration and sleep quality on game nights and the nights following game nights.

Sleep is key in the recovery process as it helps regenerate important psychological and physiological functions. Since sleep is found to be disturbed after football matches and intensive football sessions, BEMER therapy treatment may be an effective tool to improve the recovery process. Especially during international tournaments football players need tools that can improve their recovery (66). The current results are especially interesting since elite athletes normally have a well-developed microcirculation caused by the amount of aerobic training they have completed (67). The strengths of the current study are the high numbers of daily observations over an extensive period, the unique subjects who are elite level players, and the confirmation of the findings detected by the objective measurements that were used to detect sleep in the same data collection (27). The current study also has limitations that should be kept in mind. The number of participants in the current study could have been larger and a control group would have strengthened the study. The relative low number of participants and data loss might have influenced the power of the statistical analyses.