Sleep – Stressors and Cognitive Performance

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Sleep is a fundamental regulator of physiology, metabolism, and brain function [1]. Optimal health is associated with adults achieving 7 to 9 hours of sleep per night [1,2]. However, recent reviews indicate that approximately 40% of the global population experiences insufficient sleep [3].

Sleep deprivation, ranging from partial sleep deprivation (PSD) to total sleep deprivation (TSD), can lead to negative health outcomes, including increased risks of cardiovascular disease, obesity, neurodegenerative disorders, and depression [4-7]. In the short term, sleep deprivation results in immediate symptoms such as a reduction in cognitive performance, characterized by poor attention span, impaired judgment, reduced emotional capacity, and decreased cognitive flexibility [8, Van Dongen et al., 2003, Goel et al., 2009, [9,10]].

Sleep Deprivation and Other Stressors

Sleep deprivation often occurs simultaneously with other stressors. For instance, individuals visiting terrestrial high altitudes may experience disruptions to their sleep [11,12]. Additionally, the reduction in oxygen availability during acute altitude exposure can contribute to cognitive performance decrements [14-18]. Investigations in hypoxic environments have shown that individuals with poor sleep markers experience greater cognitive performance reductions [19-22].

Neurobiological Mechanisms of Sleep Deprivation and Hypoxia

The neurobiological mechanisms underlying the impact of sleep deprivation and hypoxia on cognitive performance, particularly involving the Prefrontal Cortex (PFC), remain unclear [8,23]. Both stressors appear to cause suboptimal functioning of the PFC, a region associated with executive functions [16,24].

The PFC is highly sensitive to its neurochemical environment and susceptible to stress [27-29], suggesting a potential shared neurobiological mechanism between sleep deprivation and hypoxia. However, a lack of well-designed combined stressor studies limits our understanding [30,31].

Exercise as a Cognitive Performance Enhancer

Contrary to the detrimental effects of sleep deprivation and hypoxia on cognition, moderate-intensity exercise shows promise in improving cognitive performance in both normoxic and hypoxic conditions [32-39]. Moderate intensity exercise, defined as 40-79% of maximal oxygen uptake, lasting 20 minutes to 2 hours, positively influences cognitive performance in normoxia [33] and hypoxia when severity is minimal-to-moderate [38].

The mechanisms underlying this improvement, such as the catecholamine hypothesis, arousal/”inverted u” theory, and interoceptive model, remain elusive due to methodological disparities [16,41-43]. However, acute moderate-intensity exercise consistently enhances tasks requiring executive function associated with the PFC [Baso & Suzuki, 2017].

Research Objectives and Hypotheses

Given the potential for impaired cognitive performance following insufficient sleep and exposure to hypoxic environments, this study aims to explore the benefits of acute exercise in various settings. The hypotheses for Experiment 1 (PSD) and Experiment 2 (TSD) are as follows:

Experiment 1 (PSD):

  • (1a) Three consecutive nights of PSD would reduce executive function at rest.
  • (1b) Regardless of sleep status, an acute bout of moderate-intensity exercise would improve executive function.

Experiment 2 (TSD):

  • (2a) One night of TSD and an acute bout of hypoxia, experienced in isolation and in combination, would reduce executive function at rest.
  • (2b) Regardless of sleep or hypoxic status, an acute bout of moderate-intensity exercise would improve executive function relative to rest in the same conditions.

Discussion

Overview of Key Findings

This chapter discusses the principal novel findings of the multi-experimental, combined stressor study investigating the effects of moderate intensity exercise on executive function after three nights of partial sleep deprivation (PSD) and the isolated and combined effects of one night of total sleep deprivation (TSD) and acute hypoxia.

Impact of Moderate Intensity Exercise on Executive Function

The most significant finding of the study is the consistent improvement in executive functions after moderate intensity exercise, irrespective of sleep or hypoxic status. While previous research has demonstrated exercise-related enhancements in executive function in normoxia and hypoxia [32-39], this study uniquely reveals that exercise may ameliorate executive function decrements after PSD, TSD, and the combination of TSD and hypoxia. Kojima et al. (2020) showed improved Stroop task performance after one night of TSD following moderate intensity exercise, suggesting a potential link between increased PFC oxygenation during exercise and cognitive benefits.

Exploratory Analysis on Cerebral Oxygenation

Contrary to the anticipated relationship between changes in task performance and cerebral oxygenation, the exploratory analysis using data from both experiments found no evidence for such a connection. Notably, the study indicates that moderate intensity exercise can enhance executive functions even in a hypoxic environment with lower cerebral oxygenation.

The inclusion of total sleep deprivation as an additional stressor provides evidence for multiple determinants of executive function during exercise, including alterations in catecholamine concentrations, psychophysiological factors, arousal, and motivation.

Effects of Three Nights of PSD on Executive Functions

Three nights of PSD resulted in inconsistent effects on executive functions. While the two-choice reaction time task showed a reduction, the logical relations and manikin task demonstrated improvement. Comparisons with previous research [68,69,10,70] suggest that the complexity of tasks and interindividual variability may contribute to these inconsistencies. The study further extends existing knowledge by revealing that executive functions improve during acute moderate intensity exercise regardless of sleep exposure, emphasizing the need for individualized consideration of sleep needs in future research.

Effects of One Night of TSD on Executive Functions (References: [8,10,75])

Consistent with expectations and previous literature [8,10,75], one night of TSD significantly reduced task performance, with throughput values decreased for six out of seven tasks. The surprising maintenance of performance in the 3-back task suggests potential cognitive fatigue in later tasks or low motivation, highlighting the importance of task order in TSD studies. Despite the severity of TSD, the lack of significant differences in the normoxic control condition raises questions about potential flooring effects and the role of motivation in task performance.

Impact of Moderate Hypoxia on Cognitive Performance

The literature surrounding the impact of moderate hypoxia on cognitive performance is complex [15,16,17,18,77,78,79,81,66,82,83]. The study’s estimated PaO2 values suggest a sufficient hypoxic dose to elicit physiological responses, but individual variability in cerebrovascular reactivity and regional blood flow may explain why some individuals maintained performance on certain tasks. Future investigations should consider simultaneous monitoring of multiple brain regions and focus on specific executive functions to clarify the nature of impairments in hypoxia.

Limitations and Considerations

Several limitations should be acknowledged, including the exclusion of individuals with adverse events, potential adaptation to insufficient sleep in some participants, and the inability to blind participants to PSD. The order and timing of tasks, along with participant motivation, may have influenced TSD task outcomes. The lack of cerebral blood flow measurements, brain scanning, and inclusion of healthy, young participants also warrant consideration in future research.

Future Directions (References: None)

Future investigations should explore neurophysiological determinants of cognitive performance after sleep deprivation and during exercise, utilizing sophisticated measurement techniques. Individualized consideration of sleep needs, task complexity, and motivational factors is crucial in interpreting executive function outcomes. The inclusion of diverse participant populations and refined methodologies will enhance the understanding of the intricate relationships between sleep, hypoxia, exercise, and cognitive performance.


reference link : https://www.sciencedirect.com/science/article/pii/S0031938423003347#sec0041

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