Does Elite Sport Degrade Sleep Quality? A Systematic Review

1.1 Objectives

To date, no attempt has been made to systematise and critique the literature on sleep quality in relation to elite sport. Given this, the present review was designed to achieve two aims: (1) to assess the structure, patterns and quality of sleep in elite athletes; and (2) to consider specific risk factors for sleep disturbance arising from the demands of elite sport. In light of points 1–4 (above), the working hypothesis was that elements of elite sport will negatively impact on sleep quality in elite athletes.

2.1 Search Strategy

Four electronic databases (SPORTDiscus, PubMed, Science Direct and Google Scholar) were systematically searched up to April 2016 using combinations of the following key words with appropriate truncation and medical subject headings (MeSH): sleep, sleep quality, insomnia, elite athletes, high-performance athletes, training, travel, competition and recovery.

2.2 Eligibility Criteria

The researchers independently assessed the eligibility of retrieved records on the basis of title and abstract. If the information was unclear, the full-text article was screened. Studies were required to meet the following inclusion criteria: (1) data were reported for participants competing at the elite level (defined as Olympic, international, professional or national); (2) data were reported for participants aged ≥18 years; (3) the study reported quantitative data on sleep outcomes; and (4) the study was published in a peer-reviewed journal as a full-text article.

2.3 Study Selection and Data Extraction

Titles and abstracts of potentially relevant articles were screened independently by two reviewers (LG and KM). Duplicates were removed and articles which did not meet the inclusion criteria were excluded. Full-text articles were assessed for eligibility by two reviewers (LG and KM). A pre-designed data extraction form was employed to collate data from individual studies including study design, participant ‘eliteness’ (see Sects. 2.4, 3.3), n size, participant sex, methodology of sleep assessment, and key outcomes and findings.

2.4 Study Quality Appraisal

Studies were critically appraised for evidence quality (principally participant selection, comparability and outcomes) using the Newcastle-Ottawa Scale (NOS) adapted for cross-sectional studies (see Electronic Supplementary Material Table S1). This scale has been employed in systematic reviews of elite athletes [45] and has established content and inter-rater reliability. Participant ‘eliteness’ for included studies was judged by applying the taxonomy of Swann et al. [46], which ranks participants on a continuum (score range 1–16), allowing categorisations from ‘semi-elite’, through ‘competitive elite’, and ‘successful elite’, to ‘world-class elite’ (see Electronic Supplementary Material Table S2).

2.5 Definitions of Sleep Terminology

3.1 Study Selection

The search strategy returned 1676 records. Of the 91 studies retained for full-text screening, we excluded 54 which did not meet the set criteria. Thirty-seven studies were therefore eligible for review.

3.2 Characteristics of Included Studies

The 37 studies were published between 2001 and 2016 and included Olympic/Commonwealth (n = 21, 57%), Paralympic (n = 4, 11%), and professional (n = 12, 32%) sports. The number of participants in the included studies ranged from 6 to 2067, with an overall age range of 18–30 years. Female athletes were under-represented generally, with only eight (21%) of the studies reporting values exclusively for women.

3.3 ‘Eliteness’ of Athletes

The application of the full Swann et al. [46] taxonomy was limited by participant descriptions within the included studies. Only a minority of studies (n = 6, 16%) reported ‘athlete level of experience’, with only one study reporting ‘athletes’ level of success’. As a result, participants were categorised using a modified taxonomy within which only ‘semi-elite’ or ‘competitive elite’ categories could be judged (see Electronic Supplementary Material Table S2). Accordingly, 20 studies (54%) were judged to have recruited ‘competitive elite’ participants.

3.4 Evidence Quality Appraisal

The evidence quality appraisal of the 37 studies can be seen in Electronic Supplementary Material Table S1. Given the diversity of study designs, reporting standards, outcome variables, and the very limited number of control participants, a meaningful calculation of risk (as odds ratios) and a subsequent meta-analysis was not possible. However, where studies reporting sleep quality outcomes used similar instruments, pooled estimates of prevalence were calculated. Relative to the objectives of this review, studies fell into three categories: (1) studies describing sleep structure and patterns; (2) studies describing sleep quality and insomnia symptomatology; and (3) studies exploring sport-related risk for sleep disturbances.

Overall, evidence quality of the selected studies was generally ‘low’ (mean NOS score = 5, standard deviation [SD] = 2), with 23 studies (62%) scoring <5 (low quality) and only two (5%) scoring >7 (good quality). Study designs employed were generally observational (n = 34, 92%), with only three (8%) of this sample employing control-group designs. Of the observational studies, 18 (49% of all studies) were cross-sectional and 14 (38% of all studies) were longitudinal. Very few studies (14%, n = 5), adequately reported participant level of performance, age, sex, sport, and level of experience. Just under half of the studies (49%, n = 18) provided a clear description of the protocol employed to measure sleep and used validated instruments, whilst adhering to measurement standards.

3.5 Sleep Structure and Patterns

3.6 Summary of Sleep Structure and Patterns

Instrumental measurements indicate that while the typical sleep duration of elite athletes may be similar to that of non-athletes, structural differences suggest a more fragmented, lower quality sleep among athletes, with most actigraphy studies reporting athlete sleep efficiencies below 90% (see Table 2). Subjective estimates of sleep duration among athletes are broadly consistent with instrumental measures. Both the duration and structure of sleep showed between-sport and sex differences; total sleep time is shorter in individual (versus team) sports, and also shorter in women.

3.7 Sleep Quality and Insomnia Symptomatology

Overall, the sleep assessments shown in Table 3 show a relatively high level of sleep complaints, with reports of sleep disturbance ranging from 13 to 70%. The low prevalence of ‘abnormal sleep’ reported by Samuels et al. [67] involved an arbitrary cut-off applied to an as-yet unvalidated scale and may not, therefore, represent a robust estimate. In the two studies which reported the prevalence of sleep disturbance by sex [68, 69], rates were highest among women. One study [69] explored this further, reporting that female athletes experienced more problems both initiating and maintaining sleep when compared with their male counterparts. Sleep quality differences between sports were also identified in this study [69], with elite French athletes from aesthetic sports reporting a significantly higher prevalence of insomnia symptoms (33%) compared with all other sports (26% for the sample overall).

Formal (PSQI) assessments of sleep quality suggest similarly high levels of insomnia-type symptoms (Table 4), with mean values at [64] or above the threshold of >5 [1, 62, 63, 65, 70]. This assumption is supported by the prevalence rates reported, with levels of significant sleep disturbance ranging from 38% of multi-sport athletes [64] to 57% of bobsleigh competitors [1]. The more conservative PSQI threshold of >8, indicative of highly disturbed sleep, also showed a relatively high prevalence, ranging from 22 to 26% [1, 63]. However, the possibility that the higher thresholds shown in Table 4 may mask more severe symptoms is suggested by Swinbourne et al. [63], who reported that 9% of elite Australian team sport athletes scored >10. Only one of the studies shown in Table 4 included control comparisons. Tsunoda et al. [62] compared the PSQI global scores of 14 international wheelchair basketball athletes (mean = 6; SD = 3) with 103 non-athletes (mean = 5; SD = 2), and found the difference significant (p < 0.05). In the same study, PSQI component score data also showed that athletes reported significantly lower subjective sleep quality and sleep efficiency, even though reported total sleep time showed no significant difference between the groups.

While the PSQI is not a diagnostic tool [66], four studies [30, 65, 68, 69] in this section used instruments validated against insomnia diagnostic criteria which allow inferences to be drawn regarding the prevalence of insomnia cases in elite athlete populations. Using the Athens Insomnia Scale (AIS—an instrument validated against the 10th revision of the International Statistical Classification of Diseases and Related Health Problems criteria for insomnia [71]), Dickinson and Hanrahan [30] reported a mean score for elite multisport athletes of 5 (range 0–16). Since scores of ≥6 indicate clinically significant insomnia symptoms, and since the reported score distribution from this study showed no significant skewness or kurtosis [30], then it can be assumed that, while the study does not report the prevalence of ≥6 scores, a high proportion of athletes must nevertheless have experienced serious insomnia symptoms. Consistent with this assumption, Dickinson and Hanrahan [30] also reported relatively high levels (for this age group) of daytime fatigue among athletes, together with consistent reports (from qualitative interviews) of nonrestorative sleep despite apparently adequate sleep durations. Using the Sleep Disorders Questionnaire (SDQ; a brief questionnaire validated against DSM-IV criteria for insomnia [72]), Lucidi et al. [68] reported that 4% of Italian Olympic athletes met diagnostic criteria for insomnia. Details of symptom chronicity, closely related to insomnia diagnosis, were provided by Schaal et al. [69], who reported a 6-month prevalence of insomnia symptoms of 22%, but a lifetime prevalence of insomnia symptoms of 27%, strongly indicating very high levels of sleep pathology within this nationally representative sample of elite French athletes.

3.8 Risk Factors for Sleep Disturbance

4.1 Sleep Patterns

This pattern of degraded sleep quality is clearly illustrated by the composite measure of sleep efficiency derived from the actigraphy studies shown in Table 2. The pooled average sleep efficiency for athletes (mean = 86; SD = 5%), is close to, and for many overlaps, the threshold value of 85%, below which insomnia symptoms are indicated [81]. These experiences are not necessarily associated with shorter sleep durations, with some studies indicating similarly recorded total sleep times in athletes and controls [50, 62]. Despite indications of degraded sleep quality among elite athletes, secondary analyses revealed no significant differences in sleep efficiency in a study comparing full-time female dancers and the Olympic athletes from the study by Leeder et al. [50] during normal training/practice [82]. Such comparisons suggest that low levels of sleep quality may extend to other populations with high levels of expertise, and may not necessarily be exclusive to elite athletes. To some degree, however, differences observed between studies in Table 2 are likely to reflect methodological inconsistencies in instrumental measurements of sleep; in particular, lengths of recording periods selected to establish normal sleep patterns, with studies ranging from 3 to 14 nights (see Table 2), and pre-selected thresholds for scoring sleep (e.g. low, medium or high).

4.2 Sleep Quality and Insomnia Symptomatology

Nevertheless, using the single PSQI sleep quality metric, approximately one third to one half of all elite athletes can be categorised as ‘poor sleepers’ (Table 4), with higher levels of insomnia symptoms (up to 70%) reported by Paralympic athletes (Table 3). Despite these apparently high levels of sleep complaints, the evidence does not unequivocally support a conclusion that elite sport per se either degrades sleep quality or drives high levels of insomnia diagnosis. Only four studies directly assessed the diagnosis of insomnia. The reported prevalence rate for clinical insomnia among Italian Olympic athletes (4%) [68] falls comfortably within the prevalence range of 3–6% for the general population aged 15–34 years in Southern Europe [83]. Moreover, values reported for the AIS among Australian elite athletes (0–16) [30], while greater than those for healthy adults (0–11) were less than those for diagnosed adult insomniacs (1–24) [84]. The one study which compared athletes and non-athletes found no between-group differences in total sleep time, but did report significantly superior sleep efficiency and sleep quality among non-athletes [62]. To an unknown extent, these outcomes may be influenced by methodological differences. However, while the demands of training, competition and travel certainly contribute to sleep disruption in elite sport, it is also likely that high-performance competitors share sleep vulnerabilities with their non-athlete peers. In general, athletes are drawn from younger high-achieving populations. Epidemiological studies show similarly high levels of sleep disturbance among young people in general, and university students in particular. In a community-based random sample of young adults (aged 18–29 years), for example, Wong and Fielding [85] report 34% scoring >5 on the PSQI, while both Lund et al. [86] and Ye et al. [87] found that 60% of university students were similarly classified by the PSQI as poor sleepers. Given such findings, the expectation, a priori, reflected in the sports science literature that “…poor sleep quality would not be likely in a young, healthy athletic population” [67] is not supported by the epidemiological evidence. Even so, cross-study comparisons do not allow for a definitive judgement to be made on whether elite athletes experience disproportionately higher overall levels of insomnia symptoms for their age. Such a judgement requires additional controlled (athlete vs non-athlete) comparisons in the sports science and medicine literature.

4.3 Risk Factors for Sleep Disturbance

The present results broadly identified periods of elevated insomnia symptom risk within training, travel and competition. However, when considered in relation to other achievement-focussed cohorts of younger adults, it does not necessarily follow that “professional sportspeople (both players and officials) face unique challenges relative to their ability to achieve sufficient sleep” [88]. For example, the high prevalence of poor sleep reported prior to competition in elite athletes is similar to that reported by professional ballet dancers prior to a premiere [89] and university students during an exam period [90]. Similarly, the relationship between rise times and total sleep time seen here for elite athletes (e.g. Sargent et al. [32]) is also seen among college students when comparing the impact of earlier (07:00) and later (10:00) class start times [91]. Furthermore, longer total sleep times reported for college students at weekends, when classes were not scheduled [91] are also consistent with studies reporting longer total sleep times on rest days among elite athletes (see Table 8). The rigorous, physical training regimens adhered to by elite athletes is a feature of elite sport which is absent among non-athlete populations. It is also relevant to note, however, that other populations which undertake high levels of physical training or practice, such as performing artists [82, 89], or active-duty military personnel [92], also exhibit high levels of disrupted sleep. The significant reductions in total sleep time and sleep efficiency that were reported during a short-term intensified training period among elite athletes in preparation for an Olympic games [53] were similar to those reported during an extended rehearsal period over 3 months in professional male and female ballet dancers in preparation for a premiere [89]. Given these similarities it is reasonable to suggest that the prevalence of insomnia symptoms in both elite athletes and other groups of younger adults may be mediated by common mechanisms.

States of hyperarousal (manifesting as pre-sleep cognitions and ruminations, stress or worry), were reported to be highly influential in mediating student sleep difficulties [86], and have also been identified as a prime cause of sleep disturbance in elite athletes prior to competition [37, 38]. Lund et al. [86], for example, report that delayed sleep onset (>30 min) was one of the most commonly reported sleep problems among students, with psychometrically assessed ‘stress’ the strongest single predictor of insomnia symptomatology. Similarly, in athlete populations both prior to competition [37, 38] and in general [69], sleep onset problems associated with stress and pre-sleep mentation predominate. Juliff et al. [38] found that the main reasons for delayed sleep onset prior to competitions were “thoughts about the competition” and “nervousness”, while Schaal et al. [69] found that aesthetic athletes, who may be especially concerned with body image and ‘perfection’ in performance, reported the highest levels of stress and sleep onset problems. It is reasonable to conclude, therefore, that cognitive and physiological arousal presents both an explanation for sleep disturbance, and a target for sleep management, among elite athletes.

4.4 Between-Sport Differences

Notwithstanding similarities between overall patterns of insomnia symptoms among elite athletes and younger non-athlete populations, it is also clear that some sports impact sleep patterns and quality more than others. Again, levels of pre-sleep arousal appear to play a strong role here, with typical average sleep latencies reported to vary from 40 min (for swimmers) to 8 min (for rugby players) [51]. For example, Schaal et al. [69] described higher levels of sleep complaints reported in aesthetic sports as a result of the ‘psychological toll’ experienced in athletes where success is based upon judgement by others, such as judges and coaches. Conversely, Erlacher et al. [37] suggested team sport athletes may experience less pre-competition anxiety when compared with individual athletes, largely due to a diffusion of responsibility among teams for their competitive outcomes. It is also relevant to note that sex could also explain some of the between-sport differences reported for sleep quality. It is a robust finding in sleep epidemiology that, for all adult age groupings, women tend to report higher levels of insomnia symptomatology than men [83, 85, 86]. Those sports showing higher proportions of female athletes (e.g. aesthetic sports) might be expected to reflect this trend. The evidence reviewed here further suggests that training schedules can contribute substantially to inter-sport differences. Where sports adopt very early training start times (e.g. swimming [33], triathlon, rowing [60]), athletes adopt correspondingly earlier bedtimes [32, 33, 60]. It appears, however, that such adjustment does not always compensate for truncated sleep. As a result, total sleep time tends to be lower and levels of pre-training fatigue and frequency of daytime napping tend to be higher in sports demanding earlier rise times [32, 51]. Training schedules which reduce total sleep time can also have an important, and seemingly paradoxical, impact on sleep structure. Therapeutically, sleep restriction (the induction of a mild state of sleep deprivation) is used in the treatment of insomnia to increase sleep need and thereby reduce sleep onset latencies and increase sleep efficiency (e.g. see Miller et al. [93]). In the actigraphy study by Leeder et al. [50], rowers, who have very early training start times, were also reported to have the lowest total sleep times when compared with canoers, divers and skaters. It is unsurprising, therefore, that rowers were also found to have the shortest sleep onset latencies, and the highest levels of sleep efficiency. Although interpreted as superior sleep quality [50], these latter findings are more likely attributable to restricted sleep.

4.5 Within-Sport Differences

Overall sleep quality and the impact of specific sleep challenges do not appear uniform across athletes. Individual responses to pre-competitive stress, circadian challenges, and late night and early morning scheduling all demonstrate similarly high levels of variance, with some athletes experiencing severe sleep disturbances, while others appear unchallenged. Fullagar et al. [59], for example, reported that, at a squad level, male elite football players experienced diminished total sleep times following late-evening league matches; however, within-squad comparisons also revealed wide individual variations in the degree of resultant sleep deprivation experienced. In this instance, it is possible that, to an unknown extent, these variations could reflect individual differences in chronotype (i.e. positions on the morningness–eveningness continuum) [94], or sleep need (i.e. position on the short sleeper–long sleeper continuum) [94]. In athletes, recent research has indicated a skew towards morning types [1, 80]. However, given the large variations in training schedules adopted across sports, it remains possible that some individual athletes may be disadvantaged by, say, very early training times. Despite the wide range of metrics and instruments employed to identify individuals with sleep pathologies in elite sport (see Table 3), the development of methodologies to explain individual differences in response to sleep challenges, or identification of ‘at risk’ sleep phenotypes remains under researched. For example, in a single study, Juliff et al. [38] suggested that measures of general sleep quality (as measured by the PSQI) were not associated with sleep disturbances experienced during competition periods. This indicates that metrics employed to assess general levels of sleep quality may not necessarily highlight ‘at risk’ phenotypes or inform targeted sleep management in scenarios when individuals are placed under stress (e.g. competition, travel and training).