Sleep Loss is Related to Unstable Stationary Balance in U.S. Army Soldiers in an Operationally Relevant Context

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Let's take a closer look at how the researchers came to these conclusions, and whether the results are reliable.

WHO: Twenty-three physically fit male U.S. Army Soldiers.

• The average age of study participants was between 28 and 36 years old.
• Most participants were relatively advanced in their careers, with no junior enlisted (E1–E4) Soldiers included in the study; almost half were junior non-commissioned officers (E5–E6).

HOW: This study was observational; the study participants were observed; they were not given treatments or medications. The study was also longitudinal; the researchers collected information on the study participants at two timepoints so changes in the participants could be examined over time.

To answer the question “Does sleep loss influence Soldier balance?", the researchers collected measurements of study participants' balance, under stable and unstable conditions, before the participants experienced sleep loss and again at the end of a 3-week training mission, when they were expected to have accumulated sleep loss. Researchers then compared Soldiers' balance before and after sleep loss to see if it had changed. To read the specifics of how this was accomplished, expand the following section:

• Measurements were taken at the very beginning of the 3-week mission, before Soldiers had any mission-related sleep loss. These measurements are considered the baseline measurements:
• They measured the participants' stationary balance (balance when standing still) under different conditions:
1. Ability to balance standing on a solid balance plate with eyes open (to mimic balance on even terrain, like a well-paved road).
2. Ability to balance standing on a solid balance plate with eyes closed.
3. Ability to balance on a foam mat on top of a balance plate with eyes open (to mimic balance on unstable or uneven terrain, such as an unpaved trail).
4. Ability to balance on a foam mat on top of a balance plate with eyes closed.
• Measurements were taken again near the end of the 3-week training mission, immediately following a night in which the participants' sleep was limited to 3 hours or less over a 36-hour shift. These measurements are considered the follow-up measurements:
• All four balance tests were repeated.
• Measurements of study participants' balance at the two timepoints, before the 3-week mission and again near the end of the 3-week mission, were compared to see if those measurements had changed over time because of sleep loss.

Next, to answer the question, “Is poor or inadequate sleep leading up to a time of sleep loss related to Soldier balance?", the researchers requested that the study participants answer a questionnaire during the baseline assessment about the quality and amount of their sleep during the 30 days leading up to the training mission. Although all study participants were invited to complete the questionnaire, only 14 provided completed responses. The researchers used the 14 complete responses to generate a subjective sleep-quality score, then used those scores to see if they could predict which Soldiers would have worse balance, first at baseline and then at the end of the 3-week training mission.

The main findings of this study were that sleep loss over the course of a 3-week training mission resulted in significant and adverse changes in Soldier stationary balance on uneven surfaces. The researchers also reported that Soldiers' poor sleep during the 30 days leading up to the 3-week training mission predicted a decrease in Soldier balance at the end of the mission.

1. Participants' balance on tests 1 and 2 (the solid balance plate) had not changed from baseline at the end of the 3-week training mission.
2. Participants' balance on test 3 (participants stood on the foam mat placed on top of the balance plate with their eyes open) had significantly decreased at the end of the 3-week mission when compared with their baseline.
3. Participants' balance on test 4 (participants stood on the foam mat placed on top of the balance plate with their eyes closed) did not change statistically from baseline. However, the differences among person-to-person test results increased considerably in test 4. In other words, the similarity among participants' performance on test 4 during baseline testing, indicating low variability, had decreased substantially when they were retested at the end of the 3-week mission, such that each individual's performance on the test was now considerably different from that of each of the other participants, indicating higher variability.
4. Sleep quality scores at baseline predicted study participants' balance at the end of the 3-week training mission. Specifically, worse sleep quality before the mission was related to worse balance at the end of the training mission.

So, what do these findings mean for service members?

The researchers state that these results suggest sleep loss may negatively influence balance, particularly under more difficult conditions, such as the rough, uneven, or rugged terrain common in military training or combat mission settings. This finding is meaningful for service members and helps to explain the relationship between sleep loss and increased risk for injury.

Study participants' balance on the balance plate and foam mat with eyes closed did not seem to change with sleep loss. However, the researchers observed an increase in the variability, or differences among the study participants, of test results at the end of the 3-week training mission. The authors interpret this finding to mean that the balance of some individuals might be affected by sleep loss more than other individuals. However, this is simply a hypothesis, an educated guess about what happened, and needs to be studied more.

Another important study observation was that poor sleep in the month leading up to the training mission seemed to predict worse balance, but only after sleep loss from the 3-week training mission had occurred. This finding suggests the amount and quality of sleep prior to periods of sleep-loss may be an important aspect of preventing both decreased balance and increased injury risk during missions when sleep loss is unavoidable. To address this, the researchers suggest the practice of sleep banking, which is sleeping more than the recommended 7 to 9 hours each night, in the 1 or 2 weeks leading up to periods of sleep loss to help “enhance resiliency to sleep loss."

What are the limitations of the study and what do they mean?

The results of a study should always be evaluated in the context of the study limitations, and there are limitations in this study that should be considered when the findings are interpreted. These limitations include the small number of study participants; differences in the time of day that balance testing occurred for baseline and follow-up; no assessment of participant level of fatigue during baseline or follow-up measurements; and not all participants completed questions about their sleep quality in the 30 days leading up to the 3-week mission. To see a more detailed discussion of these limitations, expand the section below:

1. The study sample size is small (only 23 individuals), restricted to a very specific Soldier population (elite, more advanced Soldiers in a small unit), and included no female Soldiers. While these limitations do not make the findings meaningless, the small number of study participants might have made detecting differences in balance between baseline and follow-up for tests 1 and 2 more challenging. The results might not be the same if a larger, more representative group of Soldiers had been studied. Nevertheless, these findings suggest that not even advanced, elite Soldiers are immune to the adverse effects of sleep loss. Therefore, this study supports the importance of at least 7 to 9 hours of sleep in Soldiers of all ages, ranks, and sexes.
2. Testing at baseline and follow-up was not performed at the same time of day. Study participants' balance might have been generally better at one time of the day than another, and since this factor was not kept consistent, it may have contributed to differences in the study participants' baseline and follow-up test results. Additionally, circadian rhythm, or the daily cycle of sleepiness, sleep, wake, and alertness can vary considerably among individuals. Baseline and follow-up testing performed at different times in the day means that participants were tested at different points in their circadian cycle, which may have affected study participants' sleepiness or alertness. This could have affected participants' balances differently.
3. Participant muscle fatigue was not assessed at either baseline or follow-up. Fatigue can certainly affect strength, coordination, and balance. At the end of the study, participants were finishing 3 weeks of training and may have had higher levels of muscle fatigue than when they were tested at baseline. Muscle fatigue may have also contributed to differences in the study participants' balance from baseline to follow-up.
4. Although all study participants were invited to complete the questionnaire about sleep quality before the mission, only 14 of the 23 did so. The researchers do not provide any explanation about these missing data. Had they been included, these data could have affected the study findings if the study participants who answered the questionnaire were very different, for instance, had better or worse sleep quality, from those who did not answer the questionnaire. Additionally, the type of statistical analysis performed for this part of the study can only establish a relationship between two factors; it cannot definitively determine whether one causes the other. For that reason, the relationship between sleep quality prior to sleep loss and worse balance after sleep loss is only interpreted as linked. 

In conclusion, these study findings, despite their limitations, support the importance of sleep for injury prevention. The findings of this study provide meaningful information about the relationship between sleeping less than 7 to 9 hours and injury risk. Specifically, this study suggests that adverse changes in Soldier balance occur after sleep loss, and it explores the relationship between sleep loss and musculoskeletal injury risk. Additionally, poor sleep leading up to unavoidable periods of sleep loss, such as during a mission, might put Soldiers at higher risk for lack of balance and, therefore, injury. Together, these findings support the vital nature of 7 to 9 hours of quality sleep (when possible) not only as a tenet of the Performance Triad but also for Service member injury prevention and readiness.

References:

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• Mantua J, AF Bessey, BM Ritland, JA Naylor, R Chabuz, AB McKeon, VF Capaldi, and WJ Sowden. 2020. “Sleep loss is related to unstable stationary balance in U.S. Army Soldiers in an operationally-relevant context." Sleep Med 73:130–134.
• McGuine TA, JJ Greene, T Best, and G Leverson. 2001. “Balance as a predictor of ankle injuries in high school basketball players." Clin J Sport Med 10(4):239–244.
• Watson NF, MS Badr, G Belenky, DL Bliwise, OM Buxton, D Buysse, DF Dinges, et al. 2015. “Recommended amount of sleep for a healthy adult: A joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society." Sleep 38(6):843–844.