  Circadian rhythms synchronize a variety of physiological processes ranging
from neural activity and hormone secretion to sleep cycles and feeding habits.
Despite significant diurnal variation, time-of-day (TOD) is rarely recorded or
analyzed in human brain research. Moreover, sleep-wake patterns, diurnal
preferences, and daytime alertness vary across individuals, known as sleep
chronotypes. Here, we performed graph-theory network analysis on resting-state
functional MRI (rs-fMRI) data to explore topological differences in whole-brain
functional networks between morning and evening sessions (TOD effect), and
between extreme morning-type and evening-type chronotypes. To that end, 62
individuals (31 extreme morning, 31 evening-type) underwent two fMRI sessions:
about 1 hour after the wake-up time (morning), and 10 hours thereafter,
scheduled in accord with their declared habitual sleep-wake pattern. TOD
significantly altered functional connectivity (FC) patterns, but there was no
significant difference in chronotypic categories. Compared to the morning
session, we found relatively increased small-worldness, modularity,
assortativity, and synchronization in the evening session, indicating more
efficient functional topology. Local measures were changed during the day
predominantly across the areas involved in somatomotor, ventral attention, as
well as default mode networks. Also, connectivity and hub analyses showed that
the somatomotor, ventral attention, and visual networks are the most
densely-connected brain areas in both sessions, respectively, with the first
being more active in the evening session and the two latter in the morning
session. Collectively, these findings suggest TOD can impact classic analyses
used in human neuroimaging such as functional connectivity, and should be
recorded and included in the analysis of functional neuroimaging data.
