Abstract
(1) Background: Global population aging is changing demographic structures and presents significant challenges for health systems, which must adapt to an increasingly elderly population. (2) Methods: The study employed a randomized controlled trial design with a total of 116 older adults aged 65 or older, randomly assigned to an experimental group (n = 57) undergoing a combined resistance program and Mediterranean diet program and a control group (n = 59) who did not receive any intervention. Anxiety and depression were evaluated using the Hospital Anxiety and Depression Scale (HADS), sleep quality through the Pittsburgh Sleep Quality Index (PSQI), and perceived stress using the Perceived Stress Scale (PSS). (3) Results: Statistically significant improvements (p < 0.05) were observed both within and between groups in anxiety (Cohen's d = 0.38 and 0.78, respectively), depression (Cohen's d = 0.56 and 0.18, respectively), perceived stress (Cohen's d = 0.15 and 0.39, respectively), and in the PSQI domains: subjective sleep quality (Cohen's d = 1.01 and 0.53, respectively), sleep duration (Cohen's d = 0.112 and 0.53, respectively), sleep disturbances (Cohen's d = 1.92 and 0.95, respectively), use of medications (Cohen's d = 0.34 and 0.40, respectively), and the PSQI total score (Cohen's d = 0.68 and 0.49, respectively). No significant differences were observed in sleep latency or daytime dysfunction. (4) Conclusions: The results of the present study suggest that resistance intervention may be an effective therapeutic option to improve mental health and sleep quality in older adults aged 65 or older, offering a non-pharmacological approach to improving overall well-being in this demographic.