Department of Electrical and Computer Engineering, N.1 Institute for Health and Memory Networks Program, National University of Singapore, Singapore.
Centre for Sleep and Cognition, Yong Loo Lin School of Medicine, National University of Singapore, Singapore.
PLoS Biol. 2020 Feb 18;18(2):e3000602. doi: 10.1371/journal.pbio.3000602. eCollection 2020 Feb.
The brain exhibits substantial diurnal variation in physiology and function, but neuroscience studies rarely report or consider the effects of time of day. Here, we examined variation in resting-state functional MRI (fMRI) in around 900 individuals scanned between 8 AM and 10 PM on two different days. Multiple studies across animals and humans have demonstrated that the brain's global signal (GS) amplitude (henceforth referred to as "fluctuation") increases with decreased arousal. Thus, in accord with known circadian variation in arousal, we hypothesised that GS fluctuation would be lowest in the morning, increase in the midafternoon, and dip in the early evening. Instead, we observed a cumulative decrease in GS fluctuation as the day progressed. Although respiratory variation also decreased with time of day, control analyses suggested that this did not account for the reduction in GS fluctuation. Finally, time of day was associated with marked decreases in resting-state functional connectivity across the whole brain. The magnitude of decrease was significantly stronger than associations between functional connectivity and behaviour (e.g., fluid intelligence). These findings reveal time of day effects on global brain activity that are not easily explained by expected arousal state or physiological artefacts. We conclude by discussing potential mechanisms for the observed diurnal variation in resting brain activity and the importance of accounting for time of day in future studies.
大脑在生理学和功能上表现出明显的昼夜变化，但神经科学研究很少报告或考虑一天中的时间影响。在这里，我们在两天内的 8 点到 10 点之间检查了大约 900 名被扫描者的静息状态功能磁共振成像（fMRI）的变化。多项动物和人类研究表明，大脑的全局信号（GS）振幅（此后称为“波动”）随着唤醒程度的降低而增加。因此，根据已知的唤醒昼夜变化，我们假设 GS 波动在早上最低，下午中期增加，傍晚下降。相反，我们观察到随着一天的进行，GS 波动呈累积性下降。尽管呼吸波动也随时间的推移而降低，但对照分析表明，这并不能解释 GS 波动的减少。最后，一天中的时间与整个大脑的静息状态功能连接的明显减少有关。减少的幅度明显强于功能连接与行为之间的关联（例如，流体智力）。这些发现揭示了全球大脑活动的时间影响，这些影响不能简单地用预期的唤醒状态或生理伪影来解释。最后，我们讨论了观察到的静息大脑活动昼夜变化的潜在机制，以及在未来研究中考虑时间的重要性。
