fMRI differences between early and late stage-1 sleep.

This study sought to test for differences in regional brain activity between stage-1 sleep immediately following wake and immediately preceding stage-2 sleep. Data were collected during daytime fMRI sessions with simultaneous EEG acquisition. A stage-1 interval was defined as follows: > or =30s of wake, immediately followed by > or =60s of continuous stage 1, immediately followed by > or =30s of stage 2. We compared brain activity between the first 30s of stage 1 (early stage 1), the last 30s of stage 1 (late stage 1), and isolated wake. A conjunction analysis sorted each voxel into one of a series of mutually exclusive categories that represented the various possible combinations of a significant increase, decrease, or no difference among these three states. The initial dataset consisted of 14 healthy volunteers. A total of 22 sessions in these participants yielded six stage-1 intervals (from four participants) that met criteria for inclusion in the analysis. There were multiple clusters of significant voxels. Examples include changes in default-mode network areas where activity increased compared to wake only in early stage 1 and a bilateral change in the hippocampus where activity increased compared to wake only in late stage 1. These results suggest that activity in anatomically identifiable, volumetric brain regions exhibit differences during stage-1 sleep that would not have been detected with the EEG. These differences may also have specific relevance to understanding the process of sleep onset as well as the neural mechanisms of performance lapses during sleep deprivation.