Beyond circadian: a yearlong prospective cohort study into human sleep using subcutaneously implanted electroencephalography

Abstract

Background: Sleep is vital for brain health and plays a key role in many neuropsychiatric conditions. Despite its importance, sleep physiology has primarily been studied in short-term laboratory settings, limiting our ability to capture ultralong-term changes in EEG-defined sleep architecture across days, weeks, and months under real-world conditions. Methods: This study leveraged a novel technology, subcutaneous electroencephalography (sqEEG) to examine ultralong-term variations in sleep. We recorded sqEEG from 20 healthy adults over a full year. We assessed intrinsic multi-day rhythms at the individual-subject level and we evaluated the effects of external factors, including season, temperature and weekends across multiple sleep metrics. Findings: Early summer was characterized by shorter total sleep time and lower REM sleep duration. Weather influenced sleep: bright sunshine was associated with shorter sleep latency, whereas greater cloud cover was associated with longer sleep latency, and higher temperatures were associated with earlier sleep timing and reduced deep sleep. Weekend nights were associated with delayed sleep onset, offset, and midpoint, along with modest changes in Sunday-night sleep architecture. Social jetlag of at least one hour was present in a meaningful minority of participants, while overall sleep regularity remained high. Robust multi-day intrinsic cycles were not detected. Interpretation: Continuous long-term sqEEG monitoring reveals that sleep is a dynamic process shaped by external environmental cues. Seasonal changes, temperature, and weekend schedules contribute to variations in sleep timing and duration throughout the year.