Variation in sleep mentation as a function of time of night.

Mentation reports collected from sleep onset. Stage 2 and REM Stage awakenings, in the first part and in the second part of the night were analyzed both with systematic psycholinguistic and global measures. Results confirm the relationship between activation and the length of sleep mentation report shown by Antrobus. Length of the report increases with sleep time, but time does not modulate qualitative inter-stage differences. By partialling out the length of the report, many inter-stage differences disappeared; however significant differences remain in the global measure of bizarreness and in the psycholinguistic measure of visual imagery. These results cannot be explained entirely by differences in attention and memory and point to more basic differences in mental activity.