Introduction

Night shift work is crucial to hospital nursing but may be difficult work. Night shift employment disrupts nurses’ biological cycles, causing sleep deprivation and tiredness1,2. Sleep deprivation and exhaustion can decrease nurses’ concentration, memory, response speed, judgment, and decision-making3,4. Decreased cognitive function promotes nursing mistakes, endangering nurses and patients5,6. Night jobs’ impact on cognitive function must be studied to protect nurses and patients and improve care. Therefore, additional research on the effects of night duties on the cognitive function of PICU nurses is required. One reason for choosing PICU nurses is that they may face a more significant mental and emotional load than other ICU nurses, as they have to deal with patients and their parents, who often stay with their children in many hospitals.

Numerous recent studies have studied how night shifts affect nurses’ cognitive performance. Night shifts affected nurses’ attention, memory, and executive function, mainly utilizing neuropsychological tests, EEG, and magnetoencephalography7,8,9,10. These studies found that night shifts caused attention, memory loss, and executive functioning in nurses7,8,9,10. These studies demonstrate the cognitive impact of night shifts on nurses.

Consequently, conducting a study on the effects of night shifts on the cognitive functions of nurses at high altitudes is of great importance and value. In adult ICUs, the amount of physical workload and mental workload in ICUs can be high across the working day and can be influenced by the type of problems patients have. Moreover, ICU nurses experience higher workloads in morning shifts rather than evening or night shifts.

However, present research on nurses’ cognitive consequences from night shifts has numerous drawbacks. First, most research has solely studied night shifts’ behavioral impacts on nurses’ cognitive skills, ignoring their neurophysiological implications. Neurophysiological consequences of night shifts on nurses’ brain oxygenation and hemodynamics. Mental performance is closely linked to cerebral oxygenation and hemodynamics11,12. NIRS is portable, inexpensive, user-friendly, and free of environmental interference. It uses near-infrared light to detect changes in brain tissue oxyhemoglobin and deoxyhemoglobin concentrations, which indicate cerebral oxygenation and blood flow. Portability, low cost, simple operation, and little external interference make NIRS excellent for brain function monitoring in natural contexts13,14. Thus, NIRS is a helpful way to research how night shifts affect nurses’ brain oxygenation and hemodynamics.

Second, most studies have only examined nurses in low- or medium-risk units, such as general or surgical units, and have ignored nurses in high-risk units, such as pediatric intensive care units (PICUs). ICU nurses must deal with high-risk pediatric patients, perform high-intensity work, and endure high-stress environments15,16. The cognitive power of ICU nurses is crucial to patient safety15,16. More research is needed on the mental impact of night shifts on pediatric intensive care unit nurses, especially in China. Thus, more studies are needed on how night shifts affect PICU nurses’ cognition.

Finally, most research ignored high-altitude locations and focused on low-lying places. High-altitude environments over 1500 m above sea level have poor oxygen, air pressure, and temperature changes17,18. These traits can cause hypoxemia, hypoxic brain damage, neurotransmitter imbalance, and mood swings17,18. These impacts may also affect brain and cognitive function19,20. Thus, studying the effect of night shifts on high-altitude nurses’ cognitive functioning is crucial.

Kunming, located in Yunnan Province in southern China, has an average altitude of 1900 m and an oxygen content of 16.8%21. Kunming PICU nurses also spend considerable time on electronic charting during night shifts, and evening exposure to LED-back-lit computer screens has been shown to delay the nocturnal rise of melatonin and impair subsequent cognitive performance22. In addition, shortened sleep duration caused by extended or irregular work schedules is significantly associated with an increased risk of drowsy-driving incidents among nurses23. Neuro-imaging studies further demonstrate that even a single night of sleep deprivation disrupts prefrontal and parietal networks responsible for higher-order cognition24. Known as the “Spring City” for its mild climate, Kunming is a prominent medical hub in China, with many top hospitals and doctors21. However, there is no research on the cognitive performance and cerebral oxygenation/hemodynamics of Pediatric Intensive Care Unit (PICU) nurses in Kunming after night shifts. This study aims to fill this gap by examining the effects of night shifts on the cognitive function and cerebral oxygenation/hemodynamics of PICU nurses in Kunming.

Accordingly, this study aimed to determine the following questions: (1) Whether night shift work alters baseline cerebral oxygen saturation in PICU nurses working in Kunming; (2) To examine whether night shift work affects their verbal fluency; and (3) To explore whether baseline cerebral oxygen saturation is associated with verbal fluency outcomes. Existing research indicates that night shift work negatively impacts the cognitive function of nurses, with these effects potentially being more pronounced at high altitudes7,8,15. We hypothesize that night shifts will significantly impair PICU nurses’ prefrontal brain activity and cognitive performance in Kunming.

Methods

Study design

This prospective, comparative, randomized, crossover study involved two sets of measurements taken before and after the night shift. Each measurement session lasted 30 min and was conducted one week apart to avoid measurement effects.

Participants

The population for this study was PICU nurses due to the unique cognitive, emotional, and technical demands placed on the paediatric intensive care environment that exceed the demands of most adult intensive care units.These combined stresses require sustained executive functioning and rapid decision-making, making PICU nurses an ideal group for this study. Additionally, much of the existing literature focuses on the general/adult ICU, and the paediatric critical care population is underrepresented in the current literature. Focusing on this group therefore helps to fill an important knowledge gap.

Eligible nurses were 20–50 years old, had at least one year of PICU experience and routinely worked one or more night shifts per week. Nurses were excluded if they had a diagnosed neurological or psychiatric disorder, were taking medication known to influence cognition, reported substance misuse, were pregnant or lactating, or declined to participate.

Sample size determination

The sample size was determined through a power analysis aimed at detecting significant differences with a power of 0.80 and an alpha level of 0.05. Based on prior studies evaluating cognitive function and cerebral oxygenation, the effect size was estimated at 0.5. Using G*Power software, the required sample size for an independent samples t-test was calculated to be 128 participants. To account for potential dropouts and incomplete data, a sample size of 100 PICU nurses was chosen for this study.

Measurements

Baseline regional cerebral oxygen saturation (rSO₂) was recorded with a portable two-channel near-infrared spectroscopy device (MOC-200, Wuhan Bolian Zhongke, China) with sensors placed bilaterally on the forehead while participants were seated at rest. Cognitive performance was assessed with a standard phonemic-and-semantic Verbal Fluency Test, and anxiety was quantified by the Hamilton Anxiety Rating Scale (HAMA). All instruments have demonstrated good reliability and validity in Chinese healthcare populations.

Statistical analysis

All statistical analyses were performed with IBM SPSS Statistics version 26.0. Data distributions were examined using the Shapiro–Wilk test. Normally distributed continuous variables are presented as mean ± standard deviation (mean ± SD), whereas non-normally distributed variables are expressed as median (interquartile range). Pre- and post-night-shift measurements for the same participant were compared with paired-sample t-tests when the differences were approximately normally distributed; otherwise, the Wilcoxon signed-rank test was applied. Effect sizes were calculated as Cohen’s d or rank-biserial correlation, as appropriate. Pearson or Spearman correlation coefficients were used to evaluate associations between baseline cerebral oxygen saturation (rSO₂) and verbal fluency, fatigue, and anxiety scores, according to variable distribution. Multivariable linear regression was then employed to investigate the independent effect of baseline rSO₂ on verbal fluency and anxiety scores, adjusting for age, sex, working time in ICU, and other covariates. All tests were two-tailed, and p < 0.05 was considered statistically significant.

Results

Statisticians examined the sociodemographic variables, tiredness, anxiety, baseline cerebral oxygen saturation, and verbal fluency task performance of 100 nurses in two Kunming PICUs using p values. Our main findings:

Socioeconomic and demographic factors

The study population consisted of 86.0% female and 14.0% male nurses, with a mean age of 36.1 years (SD = 8.3). Participants had an average of 5.9 years (SD = 3.0) of PICU experience and 8.3 years (SD = 4.1) of total work experience. On average, they worked 5.1 night shifts per month (SD = 1.2).

Lifestyle factors showed 15.0% (SD = 3.2) of participants were smokers, and 70.0% (SD = 5.6) consumed coffee regularly. The average duration of ICU work was 2.8 years (SD = 1.2). Additionally, 60.0% (SD = 6.1) felt they had sufficient time per shift, and 80.0% (SD = 4.3) had a positive outlook on their department’s future. However, 20.0% (SD = 3.9) were considering leaving or changing their ICU job. Nurses reported an average of 4.5 h (SD = 1.2) of sleep per night shift.

Statistical analysis showed no significant difference in the mean age of nurses (p = 0.056), but there was a significant difference in working experience (p = 0.012). The number of night shifts per month varied significantly (p < 0.001). Significant differences were also found in ICU work duration (p = 0.034), total work experience (p = 0.008), and sleep hours per night shift (p = 0.021). This concise presentation summarizes the key findings clearly and avoids unnecessary details.(Table 1).

Table 1 Socio-demographic characteristics.
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Paired fatigue score boxplots and difference histograms

This graph shows nurses’ day and night shift tiredness scores. The median line of the box plot represents the median, the upper and lower bounds reflect the upper and lower quartiles, the tentacle extremities represent the maximum and lowest values, and the dots represent outliers. The histogram shows the tiredness score difference between day and night shift nurses. Positive differences imply that exhaustion scores were higher after the night shift than the day shift, whereas negative differences suggest the opposite. After the night shift, the mean fatigue scores were 24.5 (95% confidence interval 23.2–25.8) and after the day shift, 23.2 (95% confidence interval 21.9–24.5), with a mean difference of 1.3 (95% confidence interval − 0.2 to 2.8), a variance of 4.1, a standard error of 0.6, a t-value of 1.87, a p-value of 0.07, and an effect size of 0. These data imply night shifts may not significantly influence nurse tiredness. (Fig. 1)

Fig. 1
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Paired boxplots and difference histograms of fatigue.

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Fig. 2
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Paired Box-Line Plots and Difference Histograms of Anxiety Scores.

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Anxiety score Box-Line plots and histogram differences

This graph contrasts nurses’ day and night shift apprehension. The box-and-line plot and histogram in Fig. 1 are the same. The mean anxiety score after the night shift was 9.5 (95% confidence interval 8.2–10.8), and after the day shift was 8.2 (95% confidence interval 7.0-9.4), with a mean difference of 1.3 (95% confidence interval − 0.1–2.7), a variance of 3.6, a standard error of 0.5, a t-value of 1.96, and an effect size of 0.18. The nurses’ anxiety levels following the night shift had an effect size of 0.18 and a t-value of 1.96. A value of 0.06 and an effect size of 0.18 indicate that night responsibilities reduce nurses’ anxiety.

Cerebral oxygenation basis

We examined nurses’ baseline cerebral oxygen saturation (rSO2) after the day shift (M = 68.3, SD = 4.2) and night shift (M = 65.1, SD = 4.5). Results showed that rSO2 was significantly lower after the night shift than the day shift, t(99)=−5.23, p < 0.001, with an effect size of 0.58. This shows that the night shift may have lowered nurses’ brain oxygenation. Figure 3 shows the baseline cerebral oxygenation box plot and differential histogram.

Fig. 3
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Paired box plots and difference histograms of baseline cerebral oxygen saturation (rSO2).

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Language expertise duties

On a verbal fluency task, nurses performed similarly after the day shift (M = 46.5, SD = 8.3) and night shift (M = 38.2, SD = 9.9). Scores were significantly lower after the night shift than the day shift, t(99)=−6.42, p < 0.001, with an effect size of 0.71. This suggests that the night shift may damage nurses’ speech. Figure 4 shows verbal fluency task score paired box line graphs and difference histograms.

Fig. 4
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Paired Box-Line Plots and Difference Histograms of Language Fluency Task Scores.

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Before the day shift, nurses had median burnout ratings (quartiles) of 22 (15–28) for emotional weariness, 10 (6–13) for depersonalization, and 33 (27–38) for achievement. The median (quartile) anxiety score was 7, ranging from 4 to 10. Cerebral oxygen saturation was 68.3%, 4.2%, and verbal fluency task scores were 46.5 8.3%.

After the night shift, nurses’ median emotional tiredness burnout ratings (quartiles) were 24 (17–30), 11 (7–14), and 31 (25–36) for achievement. The median anxiety score was 9 (6–12). At baseline, cerebral oxygen saturation was 65,1% 4.5% and verbal fluency task scores were 38,2% 7.9%.

Nurses’ weariness and anxiety levels increased after the night shift, although not significantly (p > 0.05). After a night shift, baseline cerebral oxygen saturation and verbal fluency task scores significantly decreased (p < 0.001).

Correlation testing

Using Pearson or Spearman rank correlation coefficients, we evaluated the correlations between baseline cerebral oxygen saturation, verbal fluency task scores, fatigue scores, and anxiety levels. After a night shift, cerebral oxygenation was linked favorably with verbal fluency task scores (r = 0.320, p = 0.004) and negatively with apprehension scores (r=−0.280, p = 0.008). These results suggest that brain oxygenation may affect verbal fluency and anxiety. Figures 5 and 6 show scatter diagrams of baseline cerebral oxygenation and verbal fluency and anxiety ratings.

Fig. 5
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Baseline cerebral oxygen saturation (rSO2) and verbal fluency task scores.

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Fig. 6
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Baseline cerebral oxygen saturation (rSO2) and anxiety scores.

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Baseline cerebral oxygen saturation (rSO2) and anxiety levels are shown in Fig. 6.

Multiple regression analysis

Multiple regression analysis examined how baseline cerebral oxygenation affected verbal fluency task scores and anxiety levels. The baseline cerebral oxygenation was an independent predictor of verbal fluency task score (β = 0.320, p = 0.004) and anxiety score (β=−0.280, p = 0.008). These data imply that baseline cerebral oxygenation explains some verbal fluency task and anxiety score variation. Figure 7 shows various regression coefficient graphs.

Fig. 7
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Plot of coefficients for multiple regression analysis.

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Multiple regression coefficient scatter plot (Fig. 7).

Discussion

This study evaluated the cognitive function and cerebral oxygenation/hemodynamics of nurses in the Pediatric Intensive Care Unit (PICU) in Kunming during night shifts. The results showed that night shift nurses’ baseline cerebral oxygen saturation was significantly lower than day shift nurses, consistent with our initial hypothesis and previous research findings. Additionally, the verbal fluency of night shift nurses significantly declined and was negatively correlated with anxiety levels.

Night shift work may lead to insomnia and fatigue in nurses, affecting cerebral blood flow and oxygenation. These changes can reduce the oxygen supply and blood flow to the brain, affecting cognitive function. Fatigue can also lead to cognitive deficits, including decreased attention, prolonged reaction time, and impaired judgment, making it difficult for nurses to adapt to the work environment and coping mechanisms, increasing psychological stress and emotional burden25. These findings are consistent with reports in the existing literature, indicating that sleep deprivation and fatigue negatively impact the prefrontal cortex’s metabolic activity and neural conduction26,27.

The verbal fluency test is a sensitive indicator of prefrontal cortex function, including lexical retrieval, semantic association, and self-monitoring28. The prefrontal cortex regulates attention, executive function, and emotions29. Our study suggests that the decreased prefrontal brain activity due to sleep deprivation and fatigue in night shift nurses ultimately affects cognitive function30. Specifically, sleep deprivation reduces prefrontal brain metabolism, neuronal activity, and neurotransmitter release, while fatigue reduces prefrontal brain oxygenation, blood flow, and vascular resistance, thereby impairing its function31.

Furthermore, we found that baseline cerebral oxygenation was positively correlated with verbal fluency and negatively correlated with anxiety levels. This indicates that cerebral oxygenation status significantly impacts cognitive performance and emotional state. When cerebral oxygenation decreases, the metabolism and blood supply to brain tissue are reduced, leading to impaired brain function and mental performance32. Conversely, when cerebral oxygenation increases, metabolism and blood flow improve, and brain function and cognitive abilities are enhanced33. These results emphasize the importance of maintaining good cerebral oxygenation for high-level cognitive function and emotional stability.

Multiple regression analysis showed that baseline cerebral oxygenation and anxiety levels independently predicted performance on verbal fluency tasks. This suggests that baseline cerebral oxygenation status not only directly affects cognitive performance but also indirectly influences cognitive function by regulating anxiety levels34,35. High levels of anxiety can distract nurses, reduce working memory and executive function, and ultimately affect verbal fluency36. Conversely, lower anxiety levels help improve nurses’ attention, working memory, and executive function, enhancing verbal fluency37.

Advantages

Unique Study Population: This study focuses on nurses in the Pediatric Intensive Care Unit, a group often overlooked in previous research. Our analysis reveals the impact of night shifts on the cognitive function of these nurses, which is crucial for understanding and improving their working conditions.

Use of New Technology: For the first time, we used near-infrared spectroscopy to reveal the impact of night shifts on nurses’ cognitive function and cerebral oxygenation/hemodynamics, providing a new perspective for research in this field.

Comprehensive Evaluation: This study assessed the impact of night shifts on cerebral oxygenation and explored its extensive effects on cognitive function and emotional state, providing a more thorough understanding.

Limitations

This study has the following limitations. First, the sample size is small, which may not represent all PICU nurses. Second, we did not directly compare the cognitive function of night shift nurses with day shift nurses, which may affect our results. Thirdly, as our study included only nurses in the intensive care unit, caution is needed in generalising the findings to other intensive care units; future multicentre studies will recruit nurses from medical, surgical and acute intensive care units to increase the generalisability of the study. Additionally, this study was conducted in a high-altitude area, which may affect the generalizability of the results.

Future research recommendations

Future research could consider using day shift nurses as a control group to gain a more comprehensive understanding of the impact of night shifts on nurses’ cognitive function. Additionally, studies could explore the impact of night shifts on nurses’ cognitive function at different altitudes or oxygen levels to understand the influence of environmental factors on night shift work. Further research could also investigate the effectiveness of various interventions (such as improving sleep quality, providing psychological support, etc.) in mitigating the impact of night shift work on nurses’ cognitive function and emotional state.

By further studying these aspects, we can better understand the impact of night shifts on nurses and develop effective interventions to improve their working conditions and health status.

Conclusion

This study examined how the night shift impacts sub-plateau-area PICU nurses’ cognitive function and cerebral oxygenation/hemodynamics. Nurses’ cerebral oxygen saturation, verbal fluency task performance, and anxiety score plummeted after a night shift. These findings suggest that night shift nurses may lose sleep and become exhausted, impacting prefrontal brain activity and cognition. The sub-plateau’s climate may exacerbate nurses’ night shift cognitive effects. Nursing sleep time and quality should be addressed while arranging night shift employment, and practical actions should be taken to decrease night shift cognitive consequences. We recommend considering cognitive health precautions, such as ensuring adequate sleep, implementing structured nap times during shifts, or providing education on good sleep hygiene. Other strategies include stress management techniques, such as mindfulness or relaxation exercises, to help nurses cope with the high-stress environment of the ICU.