Post by: Amanda McFarlan
What's the science?
The synaptic homeostasis theory of sleep suggests that being awake is associated with strengthening of the brain’s synapses (connections between neurons), which has a high metabolic cost, while sleep is associated with synaptic downscaling. Given that cerebral blood flow in the brain increases with greater metabolic demand, it is hypothesized that cerebral blood flow should increase during the day and decrease during sleep. Previous studies investigating cerebral blood flow have reported mixed results, so it remains unknown how sleep and wake affect cerebral blood flow. This week in NeuroImage, Elvsåshagen and colleagues investigated the effect of a day of wake, sleep and sleep deprivation on resting cerebral blood flow in the brain using neuroimaging.
How did they do it?
The authors examined resting cerebral blood flow in thirty-eight healthy adult males using arterial spin labeling (ASL) perfusion magnetic resonance imaging (MRI) to determine the effect of wake, sleep and sleep deprivation. On the first day of testing, the participants underwent ASL scanning at two time points: in the morning (after a night of sleep at home) and in the evening (approximately 14 hours after waking). The participants were then separated into two groups: the sleep group and the sleep deprivation group. The participants in the sleep group were sent home for another night of sleep, while the participants in the sleep deprivation group stayed awake all night. Both groups received a third ASL scan the following morning. Additionally, the authors interviewed participants on various aspects of their sleep habits the night, the week and the month prior to the first day of testing. The Horne-Östberg Morningness-Eveningness questionnaire and the Epworth Sleepiness Scale were used to measure participants’ chronotype and daytime sleepiness, respectively. Sleep quality was measured using the Pittsburgh Sleep Quality Index and subjective sleepiness was assessed using the Karolinska Sleepiness Scale. Participants were also asked to perform a task that measured attention by examining each individual’s variability in reaction time immediately after each ASL scan.
What did they find?
The authors compared whole-brain ASL scans taken in the morning to those taken in the evening of the first testing day and determined that resting cerebral blood flow increased bilaterally in the hippocampus, amygdala, thalamus, and sensorimotor cortices over the course of the day. However, they found no differences in resting cerebral blood flow when comparing the first morning versus the second morning in the sleep group, suggesting that resting cerebral blood flow resets after a night of sleep.
Next, the authors examined the effect of sleep deprivation on resting cerebral blood flow. In an interaction analysis, they determined that cerebral blood flow was further increased after a night of sleep deprivation in bilateral lateral and medial occipital cortices, anterior cingulate gyrus and insula, while cerebral blood flow in these areas decreased after a night of sleep. They hypothesized that the changes in resting cerebral blood flow after sleep deprivation may be explained by homeostatic mechanisms. The authors also found a positive correlation between reaction time and resting cerebral blood flow in the left and right somatosensory and motor cortices and a negative correlation between subjective sleepiness changes between the first and second mornings and cerebral blood flow in the bilateral insula. However, none of the findings related to sleep habits remained significant after being adjusted to account for multiple analyses.
What's the impact?
This is the first study to show that resting cerebral blood flow increases throughout the day in the hippocampus, amygdala, thalamus, and sensorimotor cortices, but resets after a night of sleep. Moreover, this study shows that sleep deprivation is correlated with further increases in resting cerebral blood flow in occipital and temporal cortices as well as the insula. This study provides a foundation for understanding the effect of wake and sleep on cerebral blood flow. In future studies it will be necessary to elucidate the neural mechanisms underlying these changes.
Elvsåshagen et al. Cerebral blood flow changes after a day of wake, sleep, and sleep deprivation. NeuroImage (2018). Access to the original scientific publication here.