Common Neural Correlates for Subjective and Objective Sleepiness Indices: A Functional Connectivity Study

This study examined the neural correlates in functional brain connectivity common to subjective and objective sleepiness. Because functional connectivity can be measured at rest and during tasks, it is well suited for exploring the commonalities between sleepiness during psychomotor vigilance task (PVT) and at rest with measurement of The Karolinska Sleepiness Scale (KSS). Serial resting and task-based fMRI measurements across various states of arousal may reveal a common neural substrate that does not vary with task demands. The neural basis shared by the PVT, an objective measure highly sensitive to sleep debt, and subjective reports of sleepiness may be robust markers for sleepiness and contribute to an improved understanding of the brain mechanisms underlying sleepiness.

The participants were 16 healthy right-handed (self-reported) adult men who, after a 2-week home examination, participated in a 14-day/13-night experiment that included 9 days of extended sleep (12 h per night), followed by one night of total sleep deprivation (0 h), and recovery sleep. KSS and the PVT were used as subjective and objective measures of sleepiness, respectively. Functional connectivity in the brain during each condition were measured using fMRI. In particular, the association between the inverse of the reaction time to the PVT task and resting-state functional connectivity was analyzed using a general linear mixed model.

Functional connectivity in six pairs of regions commonly associated with the KSS and PVT were identified. These included the anterior cingulate cortex-posterior cingulate cortex (part of the default mode network) and thalamus-middle temporal cortex, indicating that connectivity in these brain regions were strongly associated with sleepiness.

These results suggest a common neural substrate for subjective and objective sleepiness, which may be an important indicator of sleepiness. In addition, the functional connectivity between the thalamus and the middle temporal cortex may be a new network that deserves further attention in sleep research. The results of this study provide valuable insights into the effects of sleep deprivation and total sleep deprivation experienced in daily life on the brain and offer a new perspective on the expression of sleepiness in the brain.