Why do demanding tasks make people feel more awake even as their performance suffers? New research reveals how task difficulty and restricted sleep affect cognitive states differently.
Study: Differentiating linked cognitive states under task demand and sleep restriction: toward an experimental framework for operational monitoring. Image credit: Lena Ivanova/Shutterstock.com
Increasing task difficulty and restricting sleep affect mental workload, fatigue, drowsiness, and mind wandering in different ways, according to a new study published in Frontiers in Neuroergonomics. The findings could help researchers develop better methods for monitoring cognitive functioning in safety-critical workplaces.
Overlapping cognitive states complicate performance monitoring in demanding workplaces
Complex, dynamic real-world operational settings, such as aviation, defense, and other high-stakes industries, demand high situational awareness from operators. In these settings, even a slight instability in cognitive and attentional functioning can significantly affect human performance and safety.
Various cognitive states can influence an operator's cognitive functioning, such as mental workload, fatigue, drowsiness, and mind wandering. These cognitive states have mostly been studied in isolation to determine individual mechanisms. However, in real-world conditions, these states often overlap or emerge dynamically. This overlap complicates the differentiation of cognitive states, particularly in real-world operational settings.
The current study aimed to investigate whether these states can be differentiated through task difficulty and sleep restriction manipulations, and whether their response patterns reflect distinct causal mechanisms or a common cognitive resource. These two manipulations target complementary mechanisms, i.e., increased cognitive load and reduced alertness.
Researchers test how task difficulty and restricted sleep affect cognition
A total of 40 and 27 healthy adults participated in two independent study arms, respectively. Participants were from different high-stakes operational settings, including firefighters, police officers, and military personnel. Participants could take part in one or both study arms.
Participants in both study arms performed three cognitive tasks (multitasking, working memory, and sustained attention paradigms) under different experimental conditions targeting cognitive load and alertness. These tasks were selected to assess different aspects of cognitive functioning, including the ability to manage multiple activities, retain and process information, and maintain attention over time.
One study arm investigated the impact of two levels of task difficulty on task performance (hard session and easy session), and the other study arm assessed task performance before and immediately after approximately two hours of nighttime sleep (partial sleep restriction). In the first study arm, the easy and hard sessions were conducted on separate days, while in the second study arm, the pre-sleep and post-sleep sessions were conducted on the same day using identical task difficulty levels.
The primary aim of the study was to explore whether task difficulty and partial sleep restriction can differentiate between key cognitive states, including mental workload, drowsiness, fatigue, and mind wandering. The researchers collected behavioral performance data, subjective questionnaire responses, and physiological recordings. A key contribution of the study was the creation and initial validation of a multimodal dataset combining these measurements, which could support future research into how different cognitive states interact and can be distinguished.
Task difficulty and restricted sleep affect cognitive states differently
The researchers found that making cognitive tasks more demanding generally came at a cost to performance, but it also appeared to help participants stay more alert. In contrast, restricting sleep produced a different and less predictable pattern, affecting how participants felt more consistently than how well they performed.
As tasks became harder, participants reported greater mental workload and fatigue, particularly during activities requiring multitasking and working memory. Yet, despite these increased demands, they consistently felt less drowsy during difficult tasks than during easier ones. This seemingly contradictory finding suggests that engaging in more challenging activities may help sustain subjective alertness, even when the additional mental effort makes it harder to perform accurately.
After approximately two hours of sleep, participants reported significantly greater drowsiness, but their performance did not consistently deteriorate. Several multitasking measures improved significantly after the sleep period, while working-memory and sustained-attention performance showed no statistically significant changes. The findings suggest that feeling sleepier does not necessarily translate into an immediate, measurable decline across every type of cognitive task.
While increasing task difficulty raised perceived workload, participants reported significantly lower workload during the multitasking task after the restricted sleep period. A similar reduction emerged during the working-memory task, although it was not statistically significant. Mental fatigue, meanwhile, increased significantly as tasks became harder but showed no statistically significant change following restricted sleep.
More demanding tasks were generally associated with less mind wandering. Following restricted sleep, participants reported more mind wandering across the assessed conditions.
Together, the findings suggest that mental effort, alertness, fatigue, and mind wandering do not necessarily rise or fall together. A demanding task may leave someone feeling more mentally taxed but less sleepy, while restricted sleep may increase drowsiness without consistently impairing performance. These contrasting patterns highlight why researchers may need to examine several cognitive states together, rather than relying on a single measure to understand how people function under pressure.
Overlapping cognitive states that respond differently to demands
The study reveals that task difficulty levels exert strong and consistent effects on both cognitive task performance and self-rated cognitive states, whereas partial sleep restriction primarily influences self-rated cognitive states, with smaller, task-dependent effects on performance.
These findings highlight the significance of combining complementary cognitive manipulations to explore different dimensions of cognitive state. The observed overlap across conditions is consistent with the possibility that different manipulations selectively affect components of shared cognitive and physiological processes. However, the findings do not conclusively establish whether these response patterns arise from distinct causal mechanisms or a shared cognitive resource. The researchers identified partially distinct but overlapping response patterns.
The findings of the first study arm revealed that increasing task difficulty level from easy to hard led to poorer performance, indicated by task-dependent changes in reaction times and increased error rates. In the sustained-attention task and the communications component of the multitasking task, faster responses were accompanied by reduced accuracy or increased errors. In contrast, the working-memory task showed slower responses, reduced accuracy, and increased errors. This finding suggests a shift toward responding faster at the expense of accuracy under higher task demands in certain tasks.
The findings of the second study arm, however, showed smaller and less consistent effects of partial sleep restriction on task performance. After sleep, the multitasking task improved on several performance measures, while the working-memory and sustained-attention tasks showed no statistically significant changes. The study considered a short sleep period of about two hours, which might have had opposing effects on task performance. This brief sleep period might have provided some recovery; however, testing shortly after waking might have caused sleep inertia, temporarily impairing cognitive performance. These two opposing effects might have partially balanced each other, explaining the absence of consistent performance differences between pre- and post-sleep sessions.
In this study arm, participants performed the same tasks before and after sleep restriction during the same overnight testing protocol. Therefore, possible effects of practice or familiarization cannot be completely ruled out. Future studies may address this issue by including an appropriate control group. Other limitations include the relatively small sample size, particularly in the sleep-restriction study, and the recruitment of participants primarily from specific operational professions, which may limit the generalizability of the findings.
The study findings provide a framework for future research to develop reliable real-time systems for monitoring cognitive states in high-stakes operational settings.
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