Dreaming was thought to be confined to rapid eye movement (REM) sleep, which occurs during periods of high-frequency electroencephalographic (EEG) activity, but, more recently, dreaming has been observed during periods of non-REM sleep, which is characterized by low-frequency EEG activity. Monitoring brain activity using high-density EEG, participants were awoken during REM and non-REM sleep and reported on the presence or absence of dreaming. The authors found that the presence of high-frequency EEG activity was predictive of dreaming during non-REM sleep.