Beyond Circadian: A Yearlong Electroencephalography Study Reveals Hidden Ultralong-term Sleep Cycles

Sleep is essential for brain function and overall health. While circadian rhythms and sleep stages across the night have been well-characterized, long-term variations in sleep remain poorly understood. We used a novel technology, subcutaneous electroencephalography, to collect yearlong sleep recordings from 20 healthy individuals. We investigated intrinsic and extrinsic drivers of sleep variability and identified recurring multi-day cycles of 8-60 days in sleep duration, latency, architecture, and stability. Additionally, sleep was modulated by external factors including season, weather, weekends and holidays. This reveals that while sleep processes are governed by internal dynamics, they remain sensitive to environmental influences. These results uncover a previously unrecognized temporal variation in human sleep and suggest new directions for understanding sleep patterns and their relevance to health and disease.