Regional cerebral blood flow during light sleep--a H(2)(15)O-PET study

This is the first report on the distribution of regional cerebral blood flow (rCBF) changes during stage-1 sleep or somnolence. Two hypotheses were tested: (A) that rCBF differed between the awake relaxed state and stage-1 sleep, (B) that hypnagogic hallucinations frequently experienced at sleep onset would be accompanied by measurable changes in rCBF using positron emission tomography (PET). Eight subjects were PET-scanned with (15)O-labeled water injection in three conditions: awake, stage-1 sleep with reportable experiences and stage-1 sleep without reportable experiences. Electroencephalography (EEG) was performed continuously during the experiment. Sleep interviews were performed after each scan. The EEG was scored blindly to determine sleep stage. The sleep interviews revealed a substantial increase in how unrealistic and how leaping the thoughts were during stage-1 sleep. During sleep there was a relative flow increase in the occipital lobes and a relative flow decrease in the bilateral cerebellum, the bilateral posterior parietal cortex, the right premotor cortex and the left thalamus. Hypnagogic experiences seemed not to be associated with any relative flow changes. The topography of the occipital activation during stage-1 sleep supports a hypothesis of this state being a state of imagery. The rCBF decreases in premotor cortex, thalamus and cerebellum could be indicative of a general decline in preparedness for goal directed action during stage-1 sleep. Stage-1 sleep seems more similar to other forms of altered awareness, for example, relaxation meditation than to deeper sleep stages. We are of the opinion that stage-1 sleep represents the dreaming state of wakefulness, while rapid eye movement (REM) sleep reflects the dreaming state of the unaware, sleeping brain.