Sleep occupies more of our time than any other single activity. Given that we spend approximately one-third of our entire life asleep, sleep must have important functions for our development, our daily functioning, and our health. Generally, researchers assume that sleep serves three main functions (17). These include (i) restoring energies (recovery from daytime activities), (ii) protection from potential danger (keeping us quiet at night when our sensory capacities are limited), and (iii) learning (consolidation of important memories). As a consequence, long-term sleep deprivation and chronic sleep disturbances negatively influence humans' well-being and health (7,17). Accordingly, past research shows that good sleep is strongly associated with improved daily functioning (3), memory (34), learning capacity, and academic performance (10). There is also compelling evidence that chronic sleep disturbances adversely affect physical and psychological functioning in both adolescents (31) and adults (3). Importantly, sleep complaints and insomnia in adults seem to be on the increase worldwide (15). With prevalence estimates generally ranging from 10% to 50%, disturbed sleep seems to be a common phenomenon among young adults (21). Furthermore, women are at increased risk for experiencing sleep complaints (8).
Insomnia may have multiple causes, which include both external/behavioral (e.g., noise, uncomfortable temperature, diet, medication, physical exhaustion) and internal/psychological factors (stress, depression, anxiety disorders, pain, cognitive processes) (13). Because sleep disturbances have severe negative economic and personal implications, the prevention and reduction of sleep complaints are important public health goals (20). Daily and clinical experience in coping with poor sleep encourages the use of drugs to induce sleep (sleeping pills, alcohol, hypnotics) and to reduce sleep loss-related daytime sleepiness (caffeine). However, heightened tolerance to drugs and alcohol requires increasingly larger doses, which may result in drug-dependent sleep complaints. As an alternative, both cognitive and behavioral treatments for insomnia have proved successful (13,25). However, they generally require intervention by skilled clinical psychologists or psychiatrists, although some cognitive self-help therapy programs have proved to be successful (36).
In contrast, common opinion, together with that of physicians and sleep experts, is that exercise is an effective way of reducing sleep problems (37). Accordingly, most handbooks recommend regular exercise to cope with sleep problems, although there is advice against exercising immediately before going to sleep (36). Hence, exercise would seem to be a simple and inexpensive remedy for both the prevention and the treatment of sleep disturbances. Surprisingly, however, the scientific basis for this view is far from being established. In particular, there exists a dearth of research regarding the link between patterns of regular exercise and sleep in early adulthood. Also, gender-sensitive analyses, so far, have been more an exception than the rule.
Traditionally, to explain the impact of exercise on sleep, two potential mechanisms are invoked. First, sleep provides energy conservation, body restoration, and thermoregulatory functions (40), whereas acute exercise seems able to stabilize the circadian system and to reduce daytime sleepiness and impaired functioning associated with sleep loss. Second, exercise is associated with reduced symptoms of depression (11), anxiety (22), and lowered levels of stress (12), all of which play an important role in the onset and maintenance of disturbed sleep.
In their reviews, Youngstedt (39) and Youngstedt and Freelove-Charton (40) provide a summary of the current state of research concerning the association between exercise and sleep. Their key conclusions are that: (i) most epidemiological studies have shown an association between self-reported exercise and sleep; (ii) chronic exercise studies generally do not support sleep-promoting effects of exercise in both healthy, young normal sleepers and elderly subjects, which also applies for randomized controlled trials, although some promising findings were found with older adults experiencing insomnia or depression showing that exercise treatment elicited improvements in self-reported sleep; (iii) experimental/laboratory studies generally do not result in shorter sleep onset latency or reduced wakefulness during the night; (iv) experimental/laboratory studies point to small increases in sleep duration, slow-wave sleep, REM sleep latency, and a decrease in REM sleep duration (although the practical implications of these findings remain to be explored); and (v) field studies examining sleep in the usual environment of the participants using polysomnography or sleep logs do not provide evidence for a sleep-enhancing impact of acute exercise.
Taken together, past research suggests that a relationship between exercise and sleep is plausible but may be more complex than anticipated. With respect to the emerging relationships between exercise and sleep in epidemiological studies, many findings must be interpreted with caution. First, the internal and external validities of most investigations are questionable. Specifically, many studies may be criticized because they have applied measures with unknown psychometric properties. Second, many studies did not include accepted criteria for the diagnosis of sleep disorders, which raises doubt regarding the clinical relevance of the findings. Third, researchers have failed in the majority of all investigations to control for extraneous variables such as psychiatric symptoms to exclude plausible alternative explanations (40).
With these shortcomings in mind, the present study intended to find out how perceived physical fitness, level of exercise, and a perceived lack of physical activity are associated with insomnia, dysfunctional sleep-related thoughts, and quality of sleep among young adults when using validated instruments with good psychometric properties and after controlling for potential behavioral and psychological confounds. In addition, the present study aimed at getting further insight into the relationship between fitness, exercise, and sleep and participants' gender. The following hypotheses were advanced: First, it was hypothesized that females would report significantly more sleep complaints than males because women were consistently overrepresented among insomniacs and showed more dysfunctional presleep cognitions (hypothesis 1) (16). Second, it was assumed that increased exercise is associated with reduced sleep disturbances and increased quality of sleep (hypothesis 2) (39). This hypothesis was based on the fact that most epidemiological studies including young adults have supported the sleep-enhancing function of exercise (18,27,35). Third, it was expected that perceived lack of physical activity is related to higher insomnia scores and lower sleep quality (hypothesis 3). This presumption was because of the idea that worries about not being sufficiently active may function as possible threat cue, that is to say, perhaps individuals who believe that they are not sufficiently tired to fall asleep easily may be more inclined toward unwanted and unpleasant thoughts before sleep. These so-called dysfunctional thoughts may, in turn, constitute important risk factors for the development of sleep problems (13). Similarly, we refer to these cognitive processes to explain our fourth hypothesis that low levels of fitness will be connected with increased insomnia and impaired quality of sleep (hypothesis 4). In addition, previous research showed a link between low fitness levels and poor sleep (19). Finally, no clear-cut expectations existed whether gender moderates the relationship among exercise, fitness, and sleep.
A total of 862 students (mean ± SD = 24.67 ± 5.91 yr) from the German-speaking northwestern part of Switzerland (223 men aged 24.48 ± 4.78 yr; 639 women aged 24.74 ± 6.26 yr) took part in this study. Participants were recruited from the University of Basel (sport science: n = 250, psychology: n = 83, psychiatry: n = 22, medicine: n = 201) and from the Northwestern University of Applied Sciences (psychology: n = 66, tourism: n = 17, pedagogy: n = 127, gymnastics: n = 96). Participants received detailed information about the purpose of the study and about the voluntary basis of their participation. All participants were assured of the confidentiality of their responses and gave informed consent. Participants were required to complete several psychological, sleep, and sleep-related questionnaires as detailed below. The study was approved by the local ethical committee.
Exercise and perceived lack of exercise.
Self-reports of exercise were assessed with a slightly modified version of the Office in Motion Questionnaire, a valid and reliable instrument to measure self-reported physical activity (23). The version used in this study consisted of a list of 3 household, 8 transport, and 15 recreational activities, sexual activities, and a table for 3 additional unlisted activities. Participants were asked to indicate the frequency (0-7 d·wk−1) and duration (0-6 h per episode) for each activity during the preceding 2 wk. Based on the answers, a total weekly exercise score was calculated. Previous research has shown that the total score is moderately correlated with pedometer scores, whereas no association was found with physical fitness (23). Perceived lack of physical activity was assessed by means of a single item from the Swiss HEPA Survey 1999 asking participants whether they think they were engaged in enough physical activity to maintain good health (24).
Perceived physical fitness.
Perceived physical fitness was measured with one item ranging from 1 (very poor fitness) to 10 (excellent fitness) (30). This measure has proved to be correlated with measures of objective physical fitness and perceived well-being (29).
Insomnia is most often described as a subjective complaint of poor sleep quality or quantity despite adequate time for sleep. Acute insomnia refers to sleep problems lasting from one night to a few weeks' duration, whereas chronic insomnia refers to sleep problems lasting at least three nights weekly for at least 1 month (28). In this study, participants completed the Insomnia Severity Index (ISI) (4), which is a brief screening measure of insomnia and an outcome measure for use in treatment research. The seven items of the ISI, answered on five-point rating scales (0 = not at all, 4 = very much), refer in part to the criteria for insomnia of the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, by measuring difficulty in falling asleep, difficulties maintaining sleep, early morning awakening, increased daytime sleepiness, low daytime performance, low satisfaction with sleep, and worrying about sleep (1). The higher the overall score, the more the respondent is assumed to experience insomnia. The internal consistency of the index was good (Cronbach's α = 0.77).
Dysfunctional sleep-related thoughts.
To assess dysfunctional cognitive processes involved in the exacerbation and perpetuation of insomnia, the FEPS II (Fragebogen zur Erfassung allgemeiner Persönlichkeitsmerkmale Schlafgestörter = Questionnaire to assess personality traits of people suffering from sleep disturbances) was administered (14). The FEPS II consists of 23 items (scaled from 1 = not at all true to 5 = completely true) providing two subscales to describe levels of (i) "focusing" and (ii) "rumination." Focusing (12 items) refers to a person's tendency to continuously think about difficulties in getting to sleep, maintaining sleep, waking up early in the morning, and/or experiencing increased daytime sleepiness. Rumination (11 items) describes a person's proneness to worry about and feel preoccupied with unresolved problems. The underlying rationale for these two dimensions is that dysfunctional, negative cognitions such as continually worrying about not being able to sleep or about unresolved problems are the main factors involved in the development and persistence of sleep problems (14). The FEPS II was chosen owing to its applicability both for insomnia patients and for healthy subjects (6). Both subscales proved to be reliable tools with Cronbach's α coefficients of 0.79 for rumination and 0.81 for focusing.
Perceived quality of sleep.
To assess sleep quality, participants filled in a German adaptation (2) of the Pittsburgh Sleep Quality Index (9). The Pittsburgh Sleep Quality Index includes several indicators to assess both sleep quality and sleep disturbances. The psychometric and clinimetric properties of the instrument are convincing (5,9). The German version consisted of 22 items, of which 11 items were related to two typical weekdays and weekends (Saturdays and Sundays), respectively. The participants answered questions anchored on an eight-point Likert-type scale referring to emotional states just after waking up in the morning (three items: perceived quality of sleep, restoration, and mood), during daytime (two items: sleepiness and concentration), and before going to bed (two items: sleepiness and mood). Possible answers ranged from 1 (very bad sleep quality, not at all restored, and very bad mood) to 8 (very good sleep quality, completely restored, and very good mood). In addition, sleep onset latency (min) and the number of awakenings during nighttime were assessed. Detailed information provided about bedtime and waking up allowed to calculate a total sleep time index (h). Weekday nights were defined as nights before participants went to university or work (commonly Sunday to Thursday nights); weekend nights comprised Friday and Saturday nights. The distinction between weekday and weekend nights allowed an examination of shifts regarding bedtime between weekdays and weekend.
Pearson product-moment correlations and t-tests were calculated to find out how exercise, perceived lack of exercise, and physical fitness were associated. MANCOVA was run to test for differences in the dependent variables between subjects with varying exercise/fitness levels and gender (controlling for age, body mass index (BMI), tobacco, tee, caffeine, alcohol, drug use, and depressive symptoms).1 Exercise grouping was achieved by means of the tertiles.2 Fitness groupings were produced in the following way: 0-5 = low (n = 198), 6-8 = moderate (n = 512), and 9-10 = high (n = 150). Univariate ANCOVA was performed to get further insight into how exercise, fitness, and gender were linked to the various sleep parameters used in this study. Descriptive statistics, Cronbach's α coefficients, ANCOVA, and correlation analyses were computed using SPSS 16 (SPSS Inc., Chicago, IL).
Bivariate relationships among exercise, perceived lack of activity, physical fitness, and gender.
The correlation between perceived physical fitness and exercise was modest (r = 0.32, P < 0.001), indicating that only 10% of the variance of perceived physical fitness was attributable to exercise (R 2 = 0.102). Those participants who perceived a lack of physical activity (n = 284) reported lower levels of fitness (5.07 ± 1.87) compared with those who assumed being sufficiently active (7.61 ± 1.48, t = 466.63, P < 0.001). Significant differences between both groups were also found for self-reported exercise (t = 67.00, P < 0.001), with lower total MET scores for those lacking exercise (16.95) compared with individuals with higher involvements (25.66 ± 16.13). No significant differences (t = 0.80, P = not significant (NS)) between males (23.51 ± 15.16) and females (22.46 ± 15.19) were found for exercise participation. In turn, significant differences appeared between male (7.32 ± 1.92) and female participants (6.58 ± 2.02) with regard to perceived fitness (t = 22.93, P < 0.001). A χ2 test further showed that the percentage of individuals with a perceived lack of exercise does not vary as a function of gender (χ2 = 2.03, P = NS).
Sleep-related thoughts and insomnia.
Two (gender) × two/three (lack vs no perceived lack of physical activity; low, moderate, high fitness/exercise) MANCOVA were performed on the data. These analyses provided tests of whether gender and level of fitness/exercise had an influence on participants' dysfunctional sleep-related thoughts and insomnia. A unifactorial analysis revealed a significant multivariate main effect of gender (Wilk's λ = 0.97, F(3,835) = 9.88, P < 0.001) based on the fact that women had higher scores for insomnia (F(1,846) = 6.68, P < 0.01), focusing (F(1,846) = 19.90, P < 0.001), and rumination (F(1,846) = 24.67, P < 0.001). Multivariate analyses show that exercise (Wilk's λ = 0.99, F(6,1662) = 1.47, P = NS) did not produce significant results. In turn, both perceived lack of exercise (Wilk's λ = 0.98, F(3,823) = 5.65, P < 0.001) and perceived fitness (Wilk's λ = 0.97, F(6,1658) = 4.44, P < 0.001) turned out to be associated with significant group differences. Table 1 shows the means and SD for all three variables and for all groups. As depicted in Table 1, young adults who perceived a lack of exercise or self-rate their fitness as poor experienced more sleep disturbances, ruminated more about unresolved problems, and tended to worry more about difficulties to initiate and maintain sleep. Multivariate and univariate interaction effects could not be detected for any of the independent variables.
Sleep quality during weekdays.
A similar pattern of results emerged for weekday sleep quality. A multivariate main effect emerged for gender (Wilk's λ = 0.96, F(7,831) = 4.39, P < 0.001), indicating that women reported a lower sleep quality (P < 0.05) and felt sleepier before going to bed (P < 0.001). There was also a tendency toward lower recovery in the morning (P = 0.07) and lower mood (P = 0.06) in the evening among women. Again, multivariate analyses suggested no significant relationship if exercise was used as an independent variable (Wilk's λ = 0.99, F(14,1654) = 0.65, P = NS), whereas lack of physical activity (Wilk's λ = 0.97, F(7,819) = 3.16, P < 0.01) and fitness (Wilk's λ = 0.96, F(14,1650) = 2.66, P < 0.01) were significantly associated with sleep quality. Univariate analyses provided further insight into the nature of the multivariate effects (Table 2). Participants with high fitness levels and no perceived lack of physical activity reported higher general sleep quality (fitness only), felt more restored after awakening in the morning, and perceived lower levels of sleepiness and higher concentration during daytime. Two-way interactions between gender and fitness/exercise were not found.
Sleep quality during weekends.
With regard to sleep quality during and after the weekend nights, only fitness leads to a significant multivariate main effect (Wilk's λ = 0.97, F(14,1650) = 2.14, P < 0.01), whereas both level of exercise (Wilk's λ = 0.99, F(14,1654) = 0.85, P = NS) and lack of perceived exercise (Wilk's λ = 0.99, F(14,1650) = 1.76, P = NS) did not have any influence. Moreover, a multivariate gender effect was found (Wilk's λ = 0.98, F(7,831) = 2.13, P < 0.05). Univariate effects could be found in the sense that sleep quality and perceived recovery in the morning improved with increasing fitness (Table 3). In addition, students with a perceived lack of exercise reported lower concentration levels during daytime. Males felt less sleepy before bedtime (P < 0.05) and reported a better mood (P < 0.05). In contrast, females tended to report a lower sleep quality (P = 0.08). Significant interaction effects did not occur.
Sleep pattern during weekdays.
As our multivariate analyses point out, exercise (Wilk's λ = 0.99, F(6,1662) = 1.45, P = NS) and lack of exercise (Wilk's λ = 0.98, F(3,823) = 2.22, P = NS) were not associated with sleep pattern during weekdays. If fitness was used as predictor variable, both multivariate (Wilk's λ = 0.98, F(6,1658) = 2.88, P < 0.01) and univariate main effects occurred. Moderate and high fitness levels were connected with reduced sleep onset latency (Table 4). A multivariate gender main effect (Wilk's λ = 0.97, F(3,835) = 8.31, P < 0.001) was due to a shorter sleep duration (P < 0.001) and lower number of nocturnal awakenings (P < 0.01) among males. Male participants also tended to report a reduced sleep onset latency (P = 0.08). Significant interaction effects did not exist.
Sleep pattern during weekends.
Multivariate analyses point to the fact that exercise (Wilk's λ = 0.99, F(6,1662) = 0.67, P = NS) and gender (Wilk's λ = 0.99, F(3,835) = 1.52, P = NS) were not significantly related to sleep patterns during weekend nights. The opposite was true for perceived lack of exercise (Wilk's λ = 0.99, F(3,823) = 4.38, P < 0.001) and fitness (Wilk's λ = 0.97, F(6,1658) = 3.92, P < 0.01). Perceived lack of exercise predicts sleep pattern in the sense that students with no lack of exercise indicate a reduced duration and sleep onset latency on weekends (Table 4). A similar pattern was found for fitness. As Table 4 shows, students with high fitness levels also wake up less frequently than peers with low or moderate fitness levels. Interaction effects were not revealed.
Bedtime shift from weekdays to weekends: the influence of exercise, fitness, and gender.
Owing to the mismatch between internal circadian clock and external time (17), a shift toward increased sleep time and later bedtime on weekends may produce jet lag-like symptoms and impair quality of sleep (5). Interestingly, our findings point to higher bedtime shifts among subjects with high exercise levels, good perceived fitness, and adequate perceived exercises regimes. However, an increase in weekday-to-weekend shift was not associated with the ISI among this sample (r = −0.02, P = NS). Moreover, no gender differences were observed with regard to bedtime shift (F(1,846) = 1.52, P = NS).
Research shows that exercise is a well-accepted strategy to improve and promote falling asleep and to enhance sleep quality (35). The purpose of this study was to examine how perceived physical fitness, exercise, and a perceived lack of physical activity were related to sleep pattern, sleep-related thoughts, and sleep disturbances among young adults. The key findings in the present study can be summarized as follows: First, fitness and exercise were moderately correlated. This result is in accordance with previous studies, showing that correlations between fitness and exercise behavior are at most moderate, even if fitness is assessed by means of objective fitness testing (32). Second, participants who perceived a lack of physical activity reported lower levels of fitness and exercise involvement. Interestingly, 24.5% of the participants who did not perceive a lack of activity were in the group with the lowest exercise level. In return, 16.1% who perceived themselves as not sufficiently active figured in the group with the highest amounts of exercise. The fact that some individuals have an inaccurate perception of their exercise behavior is in line with the findings of a previous study conducted in Switzerland (24). Moreover, our first hypothesis was confirmed. Thus, strong main effects for gender were found in nearly all sleep variables, suggesting that women are at heightened risk to develop sleep disturbances, engage in dysfunctional sleep-related thoughts, and experience impaired sleep quality combined with more negatively toned emotions during daytime (16). As the other hypotheses are concerned, our findings point to the fact that the relationship between exercise and sleep might be a "mental affair." Thus, the actual behavior was not related to the entire set of sleep indicators (hypothesis 2), which was also true when separate analyses were performed for physical activity (resulting from household activities and active transportation) versus exercise and sport activities. In turn, significant relationships were found for perceived lack of physical activity and most of the sleep parameters (hypothesis 3). Also, compelling evidence existed for an association between fitness and sleep (hypothesis 4). To summarize, participants with high fitness levels and no perceived lack of physical activity exhibited lower insomnia scores were less inclined to ruminate about unresolved problems and reported less excessive intrusive thoughts about sleep difficulties. Furthermore, they perceived higher sleep quality during weekdays and weekends. Likewise, they felt more restored after waking up. On weekdays, they also reported increased concentration and reduced sleepiness during daytime. In contrast, participants with low fitness levels and a perceived lack of physical activity slept longer and reported prolonged sleep onset latency during both weekdays and weekends.
These results contradicted previous studies with young adults that found positive relationships between exercise and good sleep (18,27,35) and suggest that sleep-promoting effects might be less based on behavioral patterns but rather depend on individual appraisals about being sufficiently physically active and fit. This finding is in line with cognitive models of insomnia that point to the important role of cognitive processes such as attention, perception, memory, reasoning, beliefs, attributions, and expectations in the onset and maintenance of sleep complaints (13). Because the biological plausibility of a sleep-promoting impact is still not sufficiently established, paradoxical effects of exercise are not a surprise. Kalat (17), for instance, provides an anecdotal report of one man who "suffered insomnia for months until he realized that he dreaded going to sleep because he hated waking up to go jogging. After he switched his jogging time to late afternoon, he slept without difficulty" (p. 282). Further research is needed to explore whether objective fitness measures are similarly linked to sleep indicators. Moreover, research is needed to explore whether not feeling enough physically active can be considered as a form of "erroneous" sleep-related belief, as defined by Morin et al. (26). Finally, the findings show that the relationship among exercise, fitness, and sleep was largely independent of the participant's gender. Significant two-way interactions were found for none of the study variables.
The strengths of this study were its relatively large sample size, the use of validated exercise and fitness measures, the inclusion of various instruments to assess sleep patterns, sleep-related thoughts and insomnia, and application of accepted criteria for sleep disorders. Furthermore, this study adds to the body of literature in that gender effects were examined as a possible confounding variable (as well as BMI, depression, tea, coffee, tobacco, alcohol, and drug consumption). In addition, physical fitness, exercise, and perceived lack of physical activity were analyzed separately to determine which construct is more closely connected with sleep.
Several considerations, however, argue against overgeneralization of the findings. First, participants were all students, and they were recruited in the German-speaking part of Switzerland and were not a representative sample of the Swiss general population in early adulthood. Second, one may claim that students are a rather active population and that possible ceiling-floor effects may have contributed to the lack of relation between exercise and sleep measures. Also, gender effects regarding exercise behavior are minimal in student samples (33), which is in contrast to the findings of epidemiological studies (32). Third, findings are based on cross-sectional data that preclude a causal interpretation. It might be argued that poor sleep reduces the willingness of engaging in exercise, and thus, sleep should be seen as the origin and not as the outcome of low exercise levels (40). Fourth, no objective sleep data (e.g., using sleep EEG or actigraphy) were collected, and self-reports may be more susceptible to expectancy effects (38). Likewise, no neuroendocrine (e.g., cortisol, adrenocorticotropic hormone) assessment was performed. Fifth, no information about the proximity of bedtime of exercise was gathered (40). Sixth, daylight exposure was not taken into account, although daylight exposure has a favorable impact on well-being and psychological functioning (39). Therefore, future research should include daylight exposure as a possible confounding variable.
Taken together, the findings of the present study showed that the relationship between exercise and sleep seems to be far more complex as expected in a nonclinical sample of young and healthy adults. Gender-related patterns of results suggest that women are at increased risk to report sleep complaints in early adulthood, whereas no evidence stating that gender moderates the interplay among exercise, fitness, perceived activity levels, and sleep was found.
The authors thank Klara Spalek, Daniela Beutler, David Fasler, and Patrick Winiger for data entry. The authors also thank Prof. Richard Tinning (Brisbane, Australia) for proofreading the manuscript. The study was financially supported by the Freiwillige Akademische Gesellschaft (FAG) Basel, Switzerland. All authors declare no conflicts of interest. The FAG had no influence concerning data collection, data entry, data analyses, the interpretation of the data or the writing and the submission of the manuscript, or the journal chosen for possible publication. Finally, we acknowledge that the results of the present study have not been endorsed by the American College of Sports Medicine.
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