Human Brain Connectivity Shows 20% Seasonal Fluctuation, Peaking in Autumn and Lowest in Summer

Recent observations highlight a notable seasonal rhythm in human brain function, with activity and connectivity reaching their peak in autumn and declining to their lowest levels in summer. This phenomenon was recently brought to attention by Nicholas Fabiano, MD, who stated on social media, ">Brain activity & connectivity varies by season. Highest in the fall & lowest in the summer." This insight is supported by emerging scientific research exploring the brain's adaptation to environmental shifts throughout the year.

A comprehensive study published in Scientific Reports in 2023, analyzing fMRI data from 410 healthy adults, confirmed these seasonal patterns. Researchers found that global functional connectivity (FC) strength in the brain was highest in autumn and lowest in summer, with the highest FC strength approximately 20% greater than the lowest observed value. Specific brain networks, including the sensorimotor, posterior insula, and ventral default mode network (DMN), exhibited significant seasonal variations, consistently showing peak activity during the autumn months.

Further research, including a 2016 study in the Proceedings of the National Academy of Sciences (PNAS), has also demonstrated seasonality in human cognitive brain responses. This study revealed that while sustained attention brain responses peaked around summer and were lowest in winter, working memory tasks showed maximal responses around autumn and minimal around spring. These findings suggest a complex, process-specific impact of seasons on different cognitive functions, indicating that not all brain functions follow the same seasonal rhythm.

Scientists suggest that environmental factors, particularly day length (photoperiod) and temperature, play a significant role in these observed brain changes. The 2023 Scientific Reports study highlighted a correlation between seasonal brain activity measures and these environmental parameters. Understanding these natural fluctuations in brain activity and connectivity could have important implications for neuroimaging research, clinical diagnoses, and the development of optimal treatment strategies for conditions like seasonal affective disorder (SAD).