AIMS/BACKGROUND: Seasonal fluctuations in environmental conditions, particularly variations in photoperiod and ambient temperature, influence diverse physiological and behavioral processes. With advances in functional magnetic resonance imaging (fMRI), researchers have begun to investigate how such environmental factors modulate intrinsic brain activity. This review synthesizes current evidence on how seasonal, circadian, and environmental variations affect brain functional connectivity. METHODS: A comprehensive search of PubMed/Medical Literature Analysis and Retrieval System Online (MEDLINE), Scopus, and Web of Science databases was conducted for peer-reviewed fMRI studies published between 2010 and 2025. Eligible studies investigated the impact of environmental factors, such as photoperiod, temperature, and time of day, on brain functional connectivity. Due to methodological heterogeneity, a narrative synthesis approach was used in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines. The review process is illustrated using a PRISMA flow diagram, and a checklist is provided as supplementary material. RESULTS: Sixteen original studies spanning cross-sectional and longitudinal designs met the inclusion criteria. Seasonal and environmental exposures modulated connectivity in major networks, particularly the default mode network (DMN), salience network (SN), and sensorimotor network (SMN). For the DMN and SMN, photoperiod consistently affected connectivity, with peaks in autumn and reductions in summer. Circadian and temperature effects were also observed. Methodological factors, including scanner stability, emerged as critical. CONCLUSION: Environmental rhythms significantly affect brain connectivity and should be considered in neuroimaging research and clinical practice. These effects are sometimes comparable to clinical conditions. Future work should enhance control of environmental exposures and incorporate physiological markers to improve reproducibility. SYSTEMATIC REVIEW REGISTRATION: PROSPERO (CRD420261390073).
Naif A. Majrashi (2026) studied this question.