Time of Day Affects Global Brain Fluctuations
SINGAPORE (Dispatches) -- As the day progresses, the strength of the brain’s global signal fluctuation shows an unexpected decrease, according to a new study.
Circadian rhythms govern diverse aspects of physiology including sleep/wake cycles, cognition, gene expression, temperature regulation, and endocrine signaling. But despite the clear influence of circadian rhythms on physiology, most studies of brain function do not report or consider the impact of time of day on their findings.
To address this gap in knowledge, the team analysed functional magnetic resonance imaging (fMRI) data of approximately 900 subjects who were scanned between 8 am and 10 pm on two different days as part of the Human Connectome Project. Multiple studies have shown that the brain’s global signal fluctuates more strongly when one is drowsy (e.g. after insufficient sleep), and fluctuates less when one is more alert (e.g. after coffee). Based on known circadian variation in sleepiness, the authors hypothesized that global signal fluctuation would be lowest in the morning, increase in the mid-afternoon and dip in the early evening.
Instead, they observed a cumulative decrease in global signal fluctuation as the day progressed. This global decrease was most prominent in visual and somatosensory brain regions, which are known for expressing dynamic fluctuations within individuals over time. Across the whole brain, time of day was also associated with marked decreases in resting-state functional connectivity -- the correlated activity between different brain regions when no explicit task is being performed.
Circadian rhythms govern diverse aspects of physiology including sleep/wake cycles, cognition, gene expression, temperature regulation, and endocrine signaling. But despite the clear influence of circadian rhythms on physiology, most studies of brain function do not report or consider the impact of time of day on their findings.
To address this gap in knowledge, the team analysed functional magnetic resonance imaging (fMRI) data of approximately 900 subjects who were scanned between 8 am and 10 pm on two different days as part of the Human Connectome Project. Multiple studies have shown that the brain’s global signal fluctuates more strongly when one is drowsy (e.g. after insufficient sleep), and fluctuates less when one is more alert (e.g. after coffee). Based on known circadian variation in sleepiness, the authors hypothesized that global signal fluctuation would be lowest in the morning, increase in the mid-afternoon and dip in the early evening.
Instead, they observed a cumulative decrease in global signal fluctuation as the day progressed. This global decrease was most prominent in visual and somatosensory brain regions, which are known for expressing dynamic fluctuations within individuals over time. Across the whole brain, time of day was also associated with marked decreases in resting-state functional connectivity -- the correlated activity between different brain regions when no explicit task is being performed.