Night watch during REM sleep for the first-night effect

ABSTRACTWe experience disturbed sleep in a new place, and this effect is known as the first-night effect (FNE) in sleep research. We previously demonstrated that the FNE was associated with a protective night-watch system during NREM sleep in one hemisphere, which is shown as interhemispheric asymmetry in sleep depth in the default-mode network (DMN), and interhemispheric asymmetry in increased vigilance to monitor external stimuli. The present study investigated whether rapid eye movement (REM) sleep exhibited a form similar to a night-watch system during NREM sleep. First, we tested whether theta activity, which is an index of the depth of REM sleep, showed interhemispheric asymmetry in association with the FNE, by source-localizing to the DMN. However, interhemispheric asymmetry in theta activity during REM sleep was not found in association with the FNE. Next, we tested whether vigilance, as measured by evoked brain responses to deviant sounds, was increased in one hemisphere and showed interhemispheric asymmetry in association with the FNE during REM sleep. Because vigilance is different between the phasic period where rapid eye movements occur and the tonic period where rapid eye movements do not occur during REM sleep, REM sleep was split into phasic and tonic periods for measurements of evoked brain responses. While the evoked brain responses are generally small during the phasic period without the FNE, we found that the evoked brain response was significantly augmented by the FNE during the phasic period. In contrast, the evoked brain response during the tonic period did not differ by the presence of the FNE. Interhemispheric asymmetry in brain responses was not found during the phasic or tonic periods. These results suggest that a night-watch system for the FNE appears as interhemispheric asymmetry in sleep depth and vigilance during NREM sleep, but it appears as increased vigilance in both hemispheres during the phasic period, when vigilance to external stimuli is generally reduced without the FNE, during REM sleep. Therefore, a night-watch system associated with the FNE may be subserved by different neural mechanisms during NREM and REM sleep.