Exercise interventions for sleep and cognitive dysfunction in shift workers: a systematic review of randomized trials.

This systematic review synthesized evidence from randomized controlled trials (RCTs) to investigate the impact of structured exercise training on sleep and cognitive outcomes in adult shift workers.

The **interventions** included various forms of structured exercise:

Aerobic exercise

Resistance training

Combined aerobic and resistance training

High-intensity interval training (HIIT)

In-shift activity breaks

The **comparators** (control groups) in the included studies typically involved:

Usual activity

Wait-list controls

Non-exercise controls

The **outcome measures** assessed across the studies fell into two main categories:

1. **Sleep outcomes:**

* Sleep quality (subjective and objective)

* Sleep quantity (total sleep time)

* Sleep continuity (e.g., sleep efficiency, wake after sleep onset)

* Sleepiness and alertness

2. **Cognitive outcomes:**

* Cognitive performance (e.g., reaction time, short-term memory)

* Alertness

3. **Mechanistic pathways** (secondary outcomes in some studies):

* Circadian phase shifting

* Autonomic balance (assessed by heart rate variability, HRV)

* Inflammatory markers (e.g., interleukin-6, C-reactive protein)

The review also aimed to identify which intervention characteristics (e.g., type, timing, supervision) maximized benefits and what barriers existed to implementing exercise programs for shift workers.

The review included a total of **10 randomized controlled trials (RCTs)**, encompassing **420 adult shift workers**.

**Population:** Adult shift workers (aged 18 years or older) engaged in rotating shifts, permanent night shifts, or simulated shift schedules. The review included participants from any occupational sector, though 60% of the included studies were conducted in healthcare settings.

**Setting:** Studies were conducted in various settings, including laboratories and workplaces.

**Health Status:** The review did not specify general health status, but the focus was on shift workers experiencing sleep and cognitive dysfunction due to their work schedules.

The included studies utilized a variety of instruments and methods to assess sleep and cognitive outcomes, reflecting the heterogeneity of research in this field.

For **sleep outcomes**, common measures included:

**Pittsburgh Sleep Quality Index (PSQI):** A self-reported questionnaire assessing subjective sleep quality over the past month. Scores range from 0 to 21, with higher scores indicating poorer sleep quality. A score greater than 5 typically indicates poor sleep quality.

**Actigraphy:** Objective measurement of sleep-wake cycles using a wrist-worn device that records movement. This provides data on total sleep time, sleep efficiency (percentage of time in bed spent asleep), sleep latency (time to fall asleep), and wake after sleep onset (WASO).

**Polysomnography (PSG):** A comprehensive, objective sleep study conducted in a lab, which measures brain waves, oxygen levels, heart rate, breathing, and eye and leg movements during sleep. This provides detailed information on sleep stages (e.g., REM, slow-wave sleep).

**Karolinska Sleepiness Scale (KSS):** A 9-point self-reported scale used to assess subjective sleepiness at a specific moment, ranging from 1 (extremely alert) to 9 (very sleepy, fighting sleep).

**Self-reported sleep diaries:** Daily logs kept by participants to record their sleep patterns, including bedtime, wake time, sleep latency, and perceived sleep quality.

For **cognitive performance and alertness**, measures included:

**Psychomotor Vigilance Task (PVT):** A widely used test to measure sustained attention and reaction time. Participants respond to a visual stimulus as quickly as possible, and the task measures reaction time, number of lapses (long reaction times), and false starts.

Other cognitive tests for specific domains like short-term memory.

For **mechanistic pathways**, measures included:

**Circadian phase markers:** Not explicitly detailed in the abstract, but typically involve measuring melatonin levels (e.g., dim light melatonin onset, DLMO) or core body temperature rhythms.

**Heart Rate Variability (HRV):** Measured using electrocardiography (ECG) to assess the variation in time between heartbeats, reflecting autonomic nervous system balance (sympathetic vs. parasympathetic activity).

**Inflammatory markers:** Blood tests to measure levels of pro-inflammatory cytokines such as interleukin-6 (IL-6) and C-reactive protein (CRP).

The review prioritized the most clinically relevant or commonly reported measures when multiple outcomes for the same construct were available.

This study was a **systematic review of randomized controlled trials (RCTs)**. This design sits at the top of the evidence hierarchy for clinical questions, as it aims to synthesize the findings of multiple high-quality studies (RCTs) to provide a comprehensive and reliable answer.

**How they ran the study:**

1. **Protocol Registration:** The review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 statement and Cochrane recommendations. It was prospectively registered in PROSPERO (CRD420250650538), which means the researchers publicly declared their methods before starting the review, reducing the risk of selective reporting.

2. **Research Question (PICOS):** The review used the PICOS framework to define its scope:

* **P (Population):** Adult shift workers (≥18 years) in rotating, permanent night, or simulated shift schedules across any occupational sector.

* **I (Intervention):** Structured exercise (aerobic, resistance, combined, HIIT, or in-shift activity breaks).

* **C (Comparator):** Usual activity, wait-list, or non-exercise controls.

* **O (Outcomes):** Sleep quality/quantity, circadian markers, and cognitive performance.

* **S (Study Design):** Randomized Controlled Trials (RCTs).

3. **Search Strategy:** A comprehensive search was conducted across six major databases (PubMed, Scopus, Web of Science, MEDLINE, EMBASE, and Dimensions) from their inception up to January 2025. The search used a combination of keywords related to "shift work," "exercise," "sleep," "circadian rhythms," and "cognitive outcomes." This broad search aimed to capture all relevant studies.

4. **Study Selection:** Two independent reviewers (FA & SA) screened titles and abstracts, then assessed full texts for eligibility based on the predefined criteria. Disagreements were resolved by consensus, which helps to minimize bias in study inclusion.

5. **Data Extraction:** Data from eligible studies were extracted using a standardized form by two independent reviewers. This included study characteristics (author, year, country, design, setting, sample size, population details), intervention details (type, frequency, duration, intensity, supervision, timing), comparator details, specific outcome measures, and key quantitative findings (means, standard deviations, effect sizes, confidence intervals). For studies with multiple time points, the longest follow-up data was prioritized.

6. **Risk of Bias Assessment:** The methodological quality and risk of bias of the included RCTs were assessed using two tools:

* **Cochrane Risk of Bias 2.0 (RoB 2.0) tool:** This tool evaluates five domains: randomization process, deviations from intended interventions, missing outcome data, outcome measurement, and selection of reported results. Each study was classified as having low risk, some concerns, or high risk of bias. This is a robust tool for assessing internal validity.

* **PEDro scale:** An 11-item checklist assessing internal validity and statistical reporting. While Cochrane discourages reliance on summary scores from such scales, it was used descriptively to complement RoB 2.0 and facilitate comparison with other exercise reviews.

Both assessments were completed independently by two reviewers, with disagreements resolved by consensus.

7. **Data Synthesis:** Due to significant statistical heterogeneity across the included studies (differences in intervention types, outcome measures, and study populations), a formal meta-analysis (statistical pooling of results) was *not* conducted. Instead, the results were **narratively synthesized**. This means the findings were described and summarized qualitatively rather than combined mathematically.

**Why this design matters:**

**Systematic Review of RCTs:** This design provides the highest level of evidence for determining cause-and-effect relationships. By synthesizing multiple RCTs, it aims to overcome the limitations of individual studies and provide a more robust conclusion.

**Randomization:** The inclusion of only RCTs means that participants in the original studies were randomly assigned to either an exercise intervention or a control group. This is crucial because it helps ensure that, on average, the groups are similar at baseline in both known and unknown factors, making it more likely that any observed differences in outcomes are due to the intervention itself, rather than other confounding variables.

**Blinding:** While not explicitly detailed for the included studies, blinding (where participants,