Saccadic Eye Movements Attenuate Postural Sway but Less in Sleep-Deprived Young Adults

Sleep deprivation affects the performance of postural control and several other aspects related to attentional mechanisms that may alter sensory cue acquisition strategies. This study aimed to examine the possible effects of horizontal saccades and ocular fixation on a target in the performance of postural control in young adults with sleep deprivation. Twenty-six adults formed two groups, tested in two evaluations. In the first evaluation, participants slept normally on the night before. In the second evaluation, 13 participants were sleep deprived (SD) and 13 slept normally (control group [CG]) on the night before. In both evaluations, each participant stood upright as still as possible, in two experimental conditions: fixating the eye on a target and performing saccadic movement toward a target presented in two different locations (0.5 Hz). Each participant performed 3 trials in each condition, lasting 62 s each. Body oscillation was obtained in both anterior-posterior and medial-lateral directions. Results showed that SD participants swayed with a larger magnitude and higher velocity after sleep deprivation in the fixation condition. In the saccadic condition, body sway magnitude and velocity were reduced but were still larger/higher in the SD participants. Sleep deprivation deteriorates the performance of postural control. Saccadic eye movements improve postural control performance even in sleep-deprived participants but are still not sufficient to avoid postural control deterioration due to sleep deprivation.

Chronic Low Quality Sleep Impairs Postural Control in Healthy Adults

The lack of sleep, both in quality and quantity, is an increasing problem in modern society, often related to workload and stress. A number of studies have addressed the effects of acute (total) sleep deprivation on postural control. However, up to date, the effects of chronic sleep deficits, either in quantity or quality, have not been analyzed. Thirty healthy adults participated in the study that consisted of registering activity with a wrist actigraph for more than a week before performing a series of postural control tests. Sleep and circadian rhythm variables were correlated and the sum of activity of the least active 5-h period, L5, a rhythm variable, obtained the greater coefficient value with sleep quality variables (wake after sleep onset WASO and efficiency sleep). Cluster analysis was performed to classify subjects into two groups based on L5 (low and high). The balance tests scores used to asses postural control were measured using Biodex Balance System and were compared between the two groups with different sleep quality. The postural tests were divided into dynamic (platform tilt with eyes open, closed and cursor) and static (clinical test of sensory integration). The results showed that during the tests with eyes closed, the group with worse sleep quality had also worse postural control performance. Lack of vision impairs postural balance more deeply in subjects with chronic sleep inefficiency. Chronic poor sleep quality impairs postural control similarly to total sleep deprivation.

Sleep-disordered breathing and daytime postural stability

OBJECTIVE

Postural stability depends on the coordination of the central nervous system with visual sense, proprioceptive and vestibular information. Sleep deprivation has been shown to affect this function. The objective of our study was to assess the effects of sleep-disordered breathing (SDB) on postural stability.

METHODS

158 subjects referred for suspected SDB had an overnight sleep study and were placed on a posturographic platform in late afternoon. This platform allows measuring the centre of pressure (CoP) oscillations and to calculate: total displacement of CoP in X and Y axes, mean speed of CoP displacement and the length as function of surface (LFS) ratio (length of CoP displacement/surface of CoP trajectory).

RESULTS

98 men and 60 women were included. Mean age±SD was 45.4±5.5 years old, body mass index (BMI) 27.5±5.6 kg/m(2) and apnoea-hypopnoea index (AHI) 13.6±16.1/h. AHI was <5/h in 64 (41%) subjects, 5-15/h in 43 (27%), 15-30/h in 30 (19%) and >30/h in 21 (13%). In patients with an AHI >5/h versus AHI <5/h, we observed an important increase in LFS (+21%, p<0.001), in XY length (+23%, p<0.001) and in mean speed (+23%, p<0.001). After controlling for age, BMI and sleepiness (Epworth) in multivariate regression models, there was a positive association between all nocturnal breathing parameters (specifically: mean SpO2, AHI, oxygen desaturation index 3% and % time with SpO2 <90%) and the main stability outcomes (all p<0.05).

CONCLUSIONS

SDB severity, especially the mean nocturnal SpO2 level, is associated with impaired daytime postural stability.

Variability in Vowel Production within and between Days

Although the acoustic variability of speech is often described as a problem for phonetic recognition, there is little research examining acoustic-phonetic variability over time. We measured naturally occurring acoustic variability in speech production at nine specific time points (three per day over three days) to examine daily change in production as well as change across days for citation-form vowels. Productions of seven different vowels (/EE/, /IH/, /AH/, /UH/, /AE/, /OO/, /EH/) were recorded at 9AM, 3PM and 9PM over the course of each testing day on three different days, every other day, over a span of five days. Results indicate significant systematic change in F1 and F0 values over the course of a day for each of the seven vowels recorded, whereas F2 and F3 remained stable. Despite this systematic change within a day, however, talkers did not show significant changes in F0, F1, F2, and F3 between days, demonstrating that speakers are capable of producing vowels with great reliability over days without any extrinsic feedback besides their own auditory monitoring. The data show that in spite of substantial day-to-day variability in the specific listening and speaking experiences of these participants and thus exposure to different acoustic tokens of speech, there is a high degree of internal precision and consistency for the production of citation form vowels.

Impaired postural balance in the morning in patients with knee osteoarthritis

Postural balance (PB) is frequently used as an outcome measure in clinical and research settings when assessing patients with knee osteoarthritis (OA). Pain and stiffness is known to affect PB, and is elevated in the morning and evening in OA patients. The aim of this study was to explore if time-of-day affects PB control in knee OA patients. Centre Of Pressure (COP) excursion was measured (100Hz) by force plate technique at selected time-points (9.00 a.m., 12.30 p.m. and 4.00 p.m.) during a single day in 32 knee OA patients aged 66.0 (10.3) years. A rigorous protocol was followed to ensure comparable testing conditions across time-points. PB control was quantified by the COP variables: velocity moment (mm(2)/s), total sway area (mm(2)), total sway length (mm) and confidence ellipse area (mm(2)). A two-way mixed-effects model showed that PB significantly improved between 9.00 a.m. and 12.30 p.m. in three out of four COP variables. The observed improvement was 11.9% (p=0.011) for velocity moment, 12.2% (p=0.011) for total sway area and 9.4% (p<0.001) for total sway length. PB appears to be impaired in the morning relative to midday in knee OA patients. Thus, it is recommended that time of assessment is standardized between sessions when assessing PB in clinical and research settings in knee OA patients.

Alterations in postural control during the world's most challenging mountain ultra-marathon

We investigated postural control (PC) effects of a mountain ultra-marathon (MUM): a 330-km trail run with 24000 m of positive and negative change in elevation. PC was assessed prior to (PRE), during (MID) and after (POST) the MUM in experienced ultra-marathon runners (n = 18; finish time = 126 ± 16 h) and in a control group (n = 8) with a similar level of sleep deprivation. Subjects were instructed to stand upright on a posturographic platform over a period of 51.2 seconds using a double-leg stance under two test conditions: eyes open (EO) and eyes closed (EC). Traditional measures of postural stability (center of pressure trajectory analysis) and stabilogram-diffusion analysis (SDA) parameters were analysed. For the SDA, a significantly greater short-term effective diffusion was found at POST compared with PRE in the medio-lateral (ML; Dxs) and antero-posterior (AP) directions (Dys) in runners (p<0.05) The critical time interval (Ctx) in the ML direction was significantly higher at MID (p<0.001) and POST (p<0.05) than at PRE in runners. At MID (p<0.001) and POST (p<0.05), there was a significant difference between the two groups. The critical displacement (Cdx) in the ML was significantly higher at MID and at POST (p<0.001) compared with PRE for runners. A significant difference in Cdx was observed between groups in EO at MID (p<0.05) and POST (p<0.005) in the ML direction and in EC at POST in the ML and AP directions (p<0.05). Our findings revealed significant effects of fatigue on PC in runners, including, a significant increase in Ctx (critical time in ML plan) in EO and EC conditions. Thus, runners take longer to stabilise their body at POST than at MID. It is likely that the mountainous characteristics of MUM (unstable ground, primarily uphill/downhill running, and altitude) increase this fatigue, leading to difficulty in maintaining balance.

The effects of sleep quality and sleep quantity on concussion baseline assessment

OBJECTIVE

Proper concussion assessment is imperative for properly caring for athletes who sustain traumatic brain injuries. Decreased sleep quality and sleep quantity affect cognition and may threaten the validity of clinical measures often used as a part of the concussion assessment. The purpose of this study was to determine if sleep quality or sleep quantity affects performance on clinical measures of concussion.

DESIGN

Prospective cohort design.

SETTING

Clinical research center.

PARTICIPANTS

One hundred fifty-five college student-athletes (57 females, 98 males; age = 18.8 ± 0.8 years; mass = 78.4 ± 19.6 kg; height = 177.4 ± 12.3 cm).

INTERVENTIONS

We performed preseason baseline testing by using a well-accepted and multifaceted protocol inclusive of neurocognition, balance performance, and symptom reporting. Information related to sleep quality and sleep quantity was also collected during preseason baseline testing.

MAIN OUTCOME MEASURES

The CNS Vital Signs battery (computerized neurocognitive test), Sensory Organization Test (computerized dynamic posturography), and a Graded Symptom Checklist (symptom evaluation) were used.

RESULTS

Subjects with a low sleep quantity the night before baseline reported both a greater number of symptoms and higher total symptom severity score. No clinically significant effects for sleep quality were observed.

CONCLUSIONS

Sleep-deprived athletes reporting for baseline testing should be rescheduled for testing after a normal night's sleep.

Circadian amplitude and homeostatic buildup rate in postural control

Postural control during quiet stance is a common everyday physical activity. Sleepiness is increasingly prevalent in our 24-h society. Yet, little research exists that quantitatively links the fluctuations in sleepiness and postural control. This study quantifies the circadian amplitude and homeostatic buildup rate in postural control. With a force plate we assessed postural control in 12 participants (21-38 years) every 2h during 24h of sustained wakefulness. The sway area was 1.39 ± 0.71 mm(2) at the circadian high around noon, and 4.02 ± 0.67 mm(2) at the circadian low around 6 am (a 189% change, p=0.02). The circadian amplitude of the sway area was therefore 2.63 mm(2). The sway area was 1.92 ± 0.64 mm(2) at the start of the 24-h period and 4.42 ± 0.69 mm(2) at the end of the period (a 130% change, p<0.001). The homeostatic buildup rate of sway area was 0.04 h(-1). The circadian- and homeostatic effects on sway variability, sway velocity, sway frequency and fractal dimension were smaller but still significant. This study found that the circadian amplitude and homeostatic buildup rate are quantifiable from posturographic data, and that they have significant impact on postural control. This finding is important because it means that one could apply the framework of the famous two-process model of sleep regulation (published by Borbély in 1982) to explain the previously reported sleepiness-related changes in postural control.

The effect of extended wake on postural control in young adults

The sleep-wake cycle is a major determinant of locomotor activity in humans, and the neural and physiological processes necessary for optimum postural control may be impaired by an extension of the wake period into habitual sleep time. There is growing evidence for such a contribution from sleep-related factors, but great inconsistency in the methods used to assess this contribution, particularly in control for circadian phase position. Postural control was assessed at hourly intervals across 14 h of extended wake in nine young adult participants. Force plate parameters of medio-lateral and anterior-posterior sway, centre of pressure (CoP) trace length, area, and velocity were assessed with eyes open and eyes closed over 3-min periods. A standard measure of psychomotor vigilance was assessed concurrently under constant routine conditions. After controlling for individual differences in circadian phase position, a significant effect of extended wake was found for anterior-posterior sway and for psychomotor vigilance. These data suggest that extended wake may increase the risk of a fall or other consequences of impaired postural control.

Time-of-day influences postural balance in older adults

BACKGROUND

Postural balance assessments are performed in both clinical and basic research settings on a daily basis. During a 24-h time span our physiology and physical performance undergo radical changes as we are influenced by the circadian rhythm. The time-of-day interaction on postural balance is unknown in older adults. The aim of this study was to investigate the time-of-day effect on postural balance in older adults.

METHODS

Center of pressure (CoP) excursion was measured (100 Hz) by force plate analysis in 34 older adults during 30 s of narrow quiet bilateral stance. Measurements were performed around 9a.m., 12.30 p.m. and 4 p.m. on the same day. Postural balance was quantified by velocity-moment, confidence ellipse area, total sway area and total sway length.

RESULTS

An overall significant time-of-day (between 9 a.m. and 4 p.m.) effect was observed for velocity-moment (mm(2)/s) 57 ± 27-65 ± 29 (p = 0.001), confidence ellipse area (mm(2)) 36 ± 16-44 ± 19 (p < 0.001), total sway area (mm(2)) 548 ± 263-627 ± 285 (p = 0.001) and total sway length (mm) 373 ± 120-379 ± 113 (p = 0.037). The variation of postural balance was mostly pronounced from midday (12.30 p.m.) toward the afternoon (4 p.m.) in all sway parameters. Specifically between 12.30 p.m. and 4 p.m. confidence ellipse area increased by 18.5%, total sway area by 17.1%, velocity-moment by 15.8% and total sway length by 4.6%. No differences were observed between 9 a.m. and 12.30 p.m. in any of the sway parameters.

CONCLUSIONS

This study demonstrates that time-of-day influences postural balance in older adults. These findings have important scientific and clinical relevance, as they imply that time-of-day should be a controlled factor when assessing postural balance in older adults.

Can a simple balance task be used to assess fitness for duty?

Human fatigue, caused by sleep loss, extended wakefulness, and/or circadian misalignment, is a major cause of workplace errors, incidents and accidents. In some industries, employees are required to undertake fitness for duty testing at the start of a shift to identify instances where their fatigue risk is elevated, so that minimisation and/or mitigation strategies can be implemented. Postural balance has been proposed as a fitness for duty test for fatigue, but it is largely untested. Therefore, the purpose of this study was to examine the impact of sleep loss, extended wakefulness and circadian phase on postural balance. Fourteen male participants spent 10 consecutive days in a sleep laboratory, including three adaptation days and eight simulated shiftwork days. To simulate a quickly rotating roster, shiftwork days were scheduled to begin 4h later each day, and consisted of a 23.3-h wake episode and a 4.7-h sleep opportunity. Every 2.5h during wake, balance was measured while standing as still as possible on a force platform with eyes open for one minute, and eyes closed for one minute. Subjective sleepiness was assessed using the Karolinska Sleepiness Scale. Core body temperature, continuously recorded with rectal thermistors, was used to determine circadian phase. For measures of postural balance and subjective sleepiness, data were analysed using three separate repeated measures ANOVA with two within-subjects factors: circadian phase (six phases) and prior wake (nine levels). For subjective sleepiness, there was a significant effect of prior wake and circadian phase. In particular, sleepiness increased as prior wake increased, and was higher during biological night-time than biological daytime. For the eyes open balance task, there was no effect of prior wake or circadian phase. For the eyes closed balance task, there was a significant effect of circadian phase such that balance was poorer during the biological night-time than biological daytime, but there was no effect of prior wake. These results indicate that postural balance may be a viable tool for assessing fatigue associated with time of day, but may not be useful for assessing fatigue associated with extended hours of wake.

Effects of increased homeostatic sleep pressure on postural control and their modulation by attentional resources

OBJECTIVE

This study aimed to determine how increased sleep pressure interferes with postural control according to the availability of attentional resources and visual input.

METHODS

Thirteen healthy young adults performed a psychomotor vigilance task and postural tasks after a night of sleep and after 25 h of sleep deprivation. Primary outcome variables were calculated from the center of pressure (CoP) displacement measured by two force plates in various cognitive load and visual state conditions.

RESULTS

Sleep deprivation increased CoP anterior-posterior range in the no cognitive load condition and decreased CoP mediolateral range and velocity in the high cognitive load conditions. Sleep deprivation effects on the mediolateral range in the eyes open high cognitive load condition were significantly correlated with its effects on the psychomotor vigilance task.

CONCLUSIONS

Sleep deprivation destabilizes postural control when attentional and sensory resources are not challenged. In high cognitive load condition, sleep loss induces a general freezing effect that seems to be modulated by the degree of impairment in psychomotor speed.

SIGNIFICANCE

This study demonstrates that sleep pressure has a destabilizing effect on postural control independently of circadian factors, therefore suggesting that sleep debt may be a significant risk factor for falls.

The influences of time-of-day and sleep deprivation on postural control

The aim of this study was to check the combined and/or dissociated influences of time-of-day and sleep deprivation on postural control. Twenty subjects participated in test sessions which took place at 6:00 am, 10:00 am, 2:00 pm and 6:00 pm either after a normal night's sleep or after a night of total sleep deprivation. Postural control was evaluated by COP surface area, LFS ratio and Romberg's index. The results showed that postural control fluctuates diurnally according to three different periods, pronounced by sleep deprivation: (1) at 6:00 am, there was no modification by sleep deprivation; (2) at 10:00 am and 2:00 pm, an interaction effect was observed for COP surface area and LFS ratio after sleep deprivation. Values of COP surface area were significantly higher (P < 0.01) following the night of sleep deprivation than after the normal night's sleep (139.36 ± 63.82 mm² vs. 221.72 ± 137.13 mm² and 143.78 ± 75.31 mm² vs. 228.65 ± 125.09 mm², respectively); (3) at 6:00 pm, the LFS ratio was higher than during the two other periods (P < 0.001) whereas COP surface area decreased to the level observed at 6:00 am. At this time-of-day, only the LFS ratio was significantly increased (P < 0.05) by the night of sleep deprivation (0.89 ± 0.14 vs. 1.03 ± 0.30). This temporal evolution in postural control does not seem to be related to any deterioration in visual input as Romberg's index (150.09 ± 97.91) was not modified, regardless of the test session.

Posturographic sleepiness monitoring

Although reduced sleep often underlies traffic and occupational accidents, convenient sleepiness testing is lacking. We show that posturographic balance testing addresses this issue, because balance testing predicts hours of wakefulness, which could facilitate sleepiness testing. Here, we equate balance scores from separate trials, blinded to the experimenter, with those recorded as a function of known and increasing time awake (i.e. during sustained wakefulness); we show, that the time awake in separate trials is posturographically measurable: positive predictive value 69%, sensitivity 56%, and specificity 96%. These results encourage further work developing posturographic sleepiness monitoring.

Postural control after a night without sleep

The present study analysed the efficiency of postural control after 12 h of nocturnal forced wakefulness using Romberg's test comprising 1 min of recording with eyes-open and 1 min of recording with eyes-closed, with a 1 min break between the two sessions. Our aim was to see if the decreased postural control efficiency after a sleepless night was unspecific (in both eyes-closed and eyes-open conditions) or selective (in only one of the conditions). A total of 55 students spent a whole night awake at our laboratory and were tested at 22:00 and 08:00 h. In general, the results showed that postural sway increased, performing the recording from eyes-open to eyes-closed condition. The statokinesigram length (SL or efficiency of the postural system) increased after the sleepless night, while in eyes-open condition, the length in function of surface (LFS or accuracy of postural control) and Romberg's index (or contribution of vision to maintain posture) significantly decreased. This could indicate that after a night without sleep, there is a slower elaboration of visual inputs in the postural control process. On the basis of these results, the effects of sleep deprivation on cognitive performance were considered from a neuropsychological point of view.