Our thalamus goes to sleep before our cerebral cortex

Magnin et al. have recorded thalamic and cortical activities simultaneously in epileptic patients chronically implanted with intracerebral electrodes to address the issue of when these two regions of the brain go to sleep. (The thalamus is the main gateway through which information from our bodies flows to the cortex.) They find, contrary to the common view, that the thalamus deactivates well before the cortex during sleep onset, leaving the cortex to spin its stories without input from the world during a hypnagogic or half-awake state during which illusions or inspirations sometimes occur, giving us the impression that it is taking us longer to get to sleep than is actually the case. Here is their abstract:

Thalamic and cortical activities are assumed to be time-locked throughout all vigilance states. Using simultaneous intracortical and intrathalamic recordings, we demonstrate here that the thalamic deactivation occurring at sleep onset most often precedes that of the cortex by several minutes, whereas reactivation of both structures during awakening is synchronized. Delays between thalamus and cortex deactivations can vary from one subject to another when a similar cortical region is considered. In addition, heterogeneity in activity levels throughout the cortical mantle is larger than previously thought during the descent into sleep. Thus, asynchronous thalamo-cortical deactivation while falling asleep probably explains the production of hypnagogic hallucinations by a still-activated cortex and the common self-overestimation of the time needed to fall asleep.