Abstract
Significant differences exist in human brain functions affected by time of day and by people’s diurnal preferences (chronotypes) that are rarely considered in brain studies. In the current study, using network neuroscience and resting-state functional MRI (rs-fMRI) data, we examined the effect of both time of day and the individual’s chronotype on whole-brain network organization. In this regard, 62 participants (39 women; mean age: 23.97 ± 3.26 years; half morning- versus half evening-type) were scanned about 1 and 10 h after wake-up time for morning and evening sessions, respectively. We found evidence for a time-of-day effect on connectivity profiles but not for the effect of chronotype. Compared with the morning session, we found relatively higher small-worldness (an index that represents more efficient network organization) in the evening session, which suggests the dominance of sleep inertia over the circadian and homeostatic processes in the first hours after waking. Furthermore, local graph measures were changed, predominantly across the left hemisphere, in areas such as the precentral gyrus, putamen, inferior frontal gyrus (orbital part), inferior temporal gyrus, as well as the bilateral cerebellum. These findings show the variability of the functional neural network architecture during the day and improve our understanding of the role of time of day in resting-state functional networks.
Highlights
IntroductionMost living organisms express a rhythmic cycle across a 24 h period (circadian rhythm) that controls several physiological processes such as sleep–wake patterns [1,2], metabolic activity [3], and body temperature [4], as well as various brain functions [5] such as attention [6], working memory [7], decision bias [8], motor [9], and visual detection [10] tasks.As well as circadian rhythms, individuals have biologically different inclinations for when to sleep and when they are at their highest alertness and energy level, which are referred to as chronotypes [11]
We found a significant increase in small-worldness from the morning to the evening session (Figure 2) at higher densities (p < 0.01, Bonferroni corrected), whereas the changes were not significant in terms of chronotypes
We employed chronotype-based paradigms and performed graphtheory based network analysis in resting-state functional MRI to explore the topological differences in whole-brain functional networks between morning and evening sessions
Summary
Most living organisms express a rhythmic cycle across a 24 h period (circadian rhythm) that controls several physiological processes such as sleep–wake patterns [1,2], metabolic activity [3], and body temperature [4], as well as various brain functions [5] such as attention [6], working memory [7], decision bias [8], motor [9], and visual detection [10] tasks.As well as circadian rhythms, individuals have biologically different inclinations for when to sleep and when they are at their highest alertness and energy level, which are referred to as chronotypes [11]. Most living organisms express a rhythmic cycle across a 24 h period (circadian rhythm) that controls several physiological processes such as sleep–wake patterns [1,2], metabolic activity [3], and body temperature [4], as well as various brain functions [5] such as attention [6], working memory [7], decision bias [8], motor [9], and visual detection [10] tasks. Effects of circadian rhythms and chronotypes on whole-brain connectivity have been examined (some cases have only considered time of day [17]), the results are often contradictory and inconsistent. Orban et al [23], contrary to the common belief that “global brain signal is low in the morning and increases in the midafternoon, and drops in the early evening”, showed that the global signal fluctuation is continuously decreasing during the day
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