Non-rapid eye movement sleep 3D maze. Participants were then trained in the maze for 4 hours and later, during the various sleep cycles of nonREM sleep, REM sleep and wakefulness, they were scanned twelve times using a PET scan during the night. The PET scan demonstrated a higher blood flow in the hippocampus during SWS/non-REM sleep due to the training from the previous day while the control group exhibited no increased blood flow and they had not received the training the prior day. The brain activity during sleep, according to this study, would show the events of the previous day do make a difference.
Rapid eye movement sleep sleepers during a REM phase is a common experimental method for obtaining dream reports; 80% of neurotypical people can give some kind of dream report under these circumstances. Sleepers awakened from REM tend to give longer more narrative descriptions of the dreams they were experiencing, and to estimate the duration of their dreams as longer. Lucid dreams are reported far more often in REM sleep. (In fact these could be considered a hybrid state combining essential elements of REM sleep and waking consciousness.) The mental events which occur during REM most commonly have dream hallmarks including narrative structure, convincingness (experiential