Independent and Joint Associations of Social Isolation and Abnormal Sleep with Functional Disability Trajectories in Middle-aged and Older Adults

Technical Summary: Independent and Joint Associations of Social Isolation and Abnormal Sleep with Functional Disability Trajectories

Problem Statement
Functional disability is a critical precursor to the loss of independence and reduced quality of life among middle-aged and older adults. While previous research has often treated functional disability as a binary or cross-sectional outcome, the actual decline is a dynamic and heterogeneous process. Some individuals maintain low disability levels, while others experience gradual or rapid deterioration. Furthermore, while social isolation and abnormal sleep duration are known risk factors for adverse health outcomes, it remains unclear in Chinese cohorts whether these factors are independently associated with long-term functional disability trajectories, and whether their coexistence identifies a higher-risk subgroup.

Methodology
This study utilized longitudinal data from the China Health and Retirement Longitudinal Study (CHARLS) spanning five waves from 2011 to 2020. The final analysis included 13,191 participants aged 45 and older.

• Outcome Measurement: Functional disability was assessed using a composite score of Basic Activities of Daily Living (ADLs) and Instrumental Activities of Daily Living (IADLs), ranging from 0 to 11. Group-based trajectory modelling (GBTM) was employed to identify distinct longitudinal patterns of functional disability.
• Exposures:
  • Social Isolation: Defined by a composite score (≥2) derived from four indicators: unmarried status, infrequent contact with children (<1/week), lack of social activity participation, and rural residence.
  • Abnormal Sleep: Defined as nighttime sleep duration of <6 hours or >9 hours, with 6–9 hours considered normal.
• Statistical Analysis: Multinomial logistic regression was used to estimate relative risk ratios (RRRs) for the identified disability trajectories, adjusting for sociodemographic characteristics, health behaviors (smoking, drinking), and health status (hypertension, dyslipidemia, diabetes, BMI). Subgroup analyses by age and sex were conducted, and sensitivity analyses were performed to test the robustness of the findings.

Key Contributions and Results
The study identified three distinct functional disability trajectories:

1. Low-stable: 83.9% of participants (n=11,062).
2. Moderate-increasing: 11.7% of participants (n=1,540).
3. High-increasing: 4.5% of participants (n=589).

Independent Associations:
After full adjustment, both social isolation and abnormal sleep were independently associated with adverse trajectories:

• Social Isolation: Associated with higher likelihoods of the moderate-increasing trajectory (RRR=1.54, 95% CI: 1.36–1.74) and high-increasing trajectory (RRR=1.61, 95% CI: 1.31–1.99).
• Abnormal Sleep: Associated with the moderate-increasing trajectory (RRR=1.37, 95% CI: 1.21–1.56) and high-increasing trajectory (RRR=1.95, 95% CI: 1.59–2.39).

Joint Associations:
Participants with both social isolation and abnormal sleep exhibited the highest relative likelihoods of adverse trajectories, indicating a cumulative risk pattern:

• Moderate-increasing: RRR=2.11 (95% CI: 1.77–2.50).
• High-increasing: RRR=3.14 (95% CI: 2.36–4.17).

Subgroup and Sensitivity Findings:

• Sex Differences: The association between abnormal sleep and adverse disability trajectories was significantly stronger in women (P for interaction = 0.014). Women with both exposures had notably higher RRRs for adverse trajectories compared to men.
• Sleep Duration Heterogeneity: Sensitivity analysis suggested that social isolation combined with short sleep (<6h) was associated with both moderate- and high-increasing trajectories, whereas the combination with long sleep (>9h) was primarily associated with the high-increasing trajectory.

Significance and Claims
The authors claim that social isolation and abnormal nighttime sleep are independently associated with adverse functional disability trajectories in middle-aged and older Chinese adults. The study highlights that the co-occurrence of these factors identifies a subgroup with the highest risk of functional decline, suggesting a cumulative risk pattern.

The paper posits that these findings support the integration of social isolation screening and sleep health assessment into aging-related health services to identify individuals at risk. It suggests that promoting social participation and sleep health may help delay functional decline. The authors acknowledge limitations, including the reliance on self-reported sleep data and the specific context of the Chinese cohort, but emphasize the value of using nationally representative longitudinal data to map the dynamic nature of functional disability and its modifiable risk factors.