Sleep is an important and complex physiological process [1]. National sleep foundation (2008) reports sleep as “food for the brain” [2]. Sleep is known to be a multidimensional concept [3] which is a “complex phenomenon that is difficult to define and measure objectively” [4]. According to the American Psychiatric Association (2000) [5], sleep quality is a “complex phenomenon involving many dimensions including perceived sleep quality, sleep response time, sleep duration, sleep efficiency, sleep disorders, sleep medication, and daytime dysfunction.” Castriotta and Lai [6] stated that the quality of sleep is determined by how the individual perceives his/her night-time habits such as the ability to sleep, the depth of sleep, and the ability to sleep without medical assistance. A reduction in the quality and/or duration of sleep brings about a plethora of physiological changes in the human body such as an increase in drowsiness, reduction in the ability to concentrate, etc. Reduced sleep leads to poor short-term memory and concentration, impaired self-control behavior, and ability to learn. Major health issues ranging from cardiovascular [7] to psychological [8] illnesses are associated with poor sleep quality. A decrease in sleep may ultimately lead to a reduction in overall lifespan [9] and many of these health-related problems may start as early as adolescence [10]. Numerous factors can significantly lower the duration of sleep to less than suggested, thereby causing a variety of effects on muscular and neurological performance, cognition, and mood [11], yet, little attention has been paid to studying the effect of deprivation of sleep on these functions [12].

Postural control (PC) or balance is one of the major functions of the human body [13,14] and is one of the means of measuring the deficit of muscular and neurological performance. It is seen as a complex motor skill that checks the interaction of many sensorimotor processes [15] with two main functional objectives: postural equilibrium and postural orientation. The body’s posture and balance depend on the interaction of the central nervous system with proprioceptive, vestibular, and visual information [16-18]. This allows for the placement of different parts of the body about the other along with the position of the body in space which makes it possible to utilize the muscles sufficiently to maintain balance, which requires a high level of control, especially when one or the other sensory input is not effective [19]. Any pathology that affects the organs particularly involved in the maintenance of posture/balance affects the PC.

It has been shown that sleep deprivation leads to deficits in cognitive functions [20] which were specifically identified as disturbances in the circadian rhythms [21]; however, a knowledge gap still exists in understanding how sleep affects various important body functions, especially those related to neuronal and muscular systems which are measured particularly via balance [12]. With ongoing research, it has become clear that sleep disorders have some effect on balance and PC [22]. In addition, falls among vulnerable populations such as the elderly [23], as well as driving accident incidents are associated with various sleep disorders and consequent balance deterioration [24], but a deeper understanding of the concepts is needed.

The objectives of this review paper were: 1) To present a narrative review that investigates the relationship between sleep parameters and PC and 2) to cast light on the scope for further research in the future regarding balance and sleep.

The depiction of the “sleep” variable was evaluated in different views, for instance 1) sleep deprivation; 2) sleep quality; and 3) sleep duration. The methodological approach was common for all studies (i.e., the quantitative approach). Table 1 displays the authors, year of publication, type of sample, and age/status. Table 2 shows the main results of the studies conducted on sleep and balance/PC.

The present study reviewed the papers that evaluated the association between various sleep parameters and PC and indicated that impairment in sleep has a negative impact on the body’s PC. This review shows that individuals who experience poor sleep, have a greater risk of developing alterations in the PC than those having an adequate amount of sleep. All the included papers pointed in the direction of a positive association between sleep and PC, i.e., any disturbance in sleep would result in deterioration of PC.

The majority of the studies focused on finding the effects of ‘acute’ sleep deprivation (ranging from 6 to 36 hours) on PC, concluding that acute sleep deprivation influences the PC negatively. Interestingly, one of the studies showed the relationship between acute sleep deprivation and PC differently, incorporating the effect of ‘positioning’ [27]. Caldwell et al. [27] examined the effects of posture on brain activity using EEG and Psychomotor Vigilance Task (PVT) performance in individuals having sleep deprivation. They concluded that the upright position increased EEG stimulation and attention, suggesting that postural manipulations could be useful in addressing the ill effects of sleep deprivation.

Out of all the papers, only one study [38] focussed on the impact of ‘chronic’ sleep deprivation on PC. The study involved the use of actigraphy and three questionnaires (HO, ESS, and PSQI) for the assessment of various sleep parameters. The posture was assessed using the Biodex Balance System (BBS, Biodex Medical Systems Inc., Shirley, NY, USA) with the postural tests being divided into dynamic and static. The study concluded that impairment in the PC due to chronic poor sleep quality occurred in a way similar to that of total sleep deprivation. Also, during the testing with closed eyes, the group having worse sleep quality exhibited poor PC performance, thus indicating that lack of vision-impaired PC to a higher extent in subjects with the inefficiency of sleep of chronic type. However, more studies are required to study the changes in the PC as a result of chronic impairment of sleep as changes in sleep over a longer period may represent a better exposure measure.

The effect of poor sleep in relation to the time of day was also studied and it was concluded that it influenced the body’s PC [33]. According to Bougard et al. [33], PC was particularly affected at mid-day (10:00 Am and 2:00 Pm) along with improved PC after a normal night of sleep. One of the studies explored the impact of aging and concluded that loss of sleep had a more disturbing effect on PC in the older-adult group than in younger participants [34]. This also indicated a high risk of falling among the elderly.

The exact mechanisms that underlie this relationship between sleep and PC have not been fully understood. According to this review, factors associated with deteriorated PC due to impaired sleep included increased postural sway [12,25,26,29,37], difficulty in maintaining upright posture [27,28], reduction of adaptation [31], feeling of fatigue [32], diurnal fluctuations of the centre of pressure (COP) surface area [33], and alteration in COP range and speed [34-36]. It was noticed that the effect of fatigue on postural performance due to sleep deprivation enhanced following the occlusion of vision [30]. An increase in the risk of falls due to extended periods of wakefulness was also observed [31,36]. The role of visual information was also studied indicating that visual cues play a key role in maintaining balance after a period of sleep deprivation [12,26,30-32,34,35,37].

There were a few limitations to the studies. The sleep assessments performed were either subjective or quantitative, depending on the tools used. The studies involved the assessment of various parameters of sleep using different tools ranging from questionnaires to instrument-based assessments. This discrepancy in the method of evaluating the different sleep parameters and cross-study outcomes could not limit variability due to differences in methods. However, as sleep is a multidimensional measure, it is difficult to limit it to only one type of assessment method. The majority of the included studies used subjective sleep assessment which enabled to provide an individual’s perception of sleep along with the evaluation of psychological disorders, thereby proving to be more useful in clinical settings [39] which is somewhat difficult with the objective assessment of sleep [40]. However, actigraphy, sleep diaries, and PSG from small-scale investigations and some large populations showed a high correlation between subjective estimates of sleep with more direct evaluations [41,42].

Since the majority of the included studies had young adults as the study participants, a good amount of heterogeneity was not present to verify the different aspects of sleep and PC in diverse populations. This heterogeneity is important as the quality of sleep and PC are capable of affecting the health of different types of populations. The associations are therefore needed to be studied in a variety of populations. It has been seen that older individuals have slower reaction time as compared to young individuals in the PVT performance irrespective of sleep deprivation, however, in the case of sleep deprivation, the older individuals show a greater decline in the reaction time till 16 hours of sleep deprivation following which, both the young and older individuals show a similar decrement in the PVT performance. This indicates that age also plays a major role in determining the level of performance in sleep-deprived individuals [43]. Only one study [34] focused on the effect of age on the changes observed in PC due to sleep deprivation indicating that sleep has a negative influence on PC.

Investigations in the majority of the included papers were done in terms of ‘sleep deprivation. No study measured the effect of ‘excessive quantity of sleep’ on the balance system. An excessive amount of sleep is found to be prevalent throughout the population and is similar to a wide range of sleep-related, neurological and mental disorders, and its effects are therefore needed to be carefully studied [44].

The results of this study can only be representative of studies that have been included and are unable to provide representative inference for all published studies, but not included. Although there was no bias in the selection process, some studies could have been missed out from the analysis. To the best of our knowledge, no review has been performed to provide an exhaustive summary of current literature related to the relationship between sleep and PC. Further research in this area should be encouraged, including individuals having chronic sleep deprivation, considering various other factors such as their age, gender, BMI, etc.