A Medical News Today report investigates how we dream and what causes nightmares.
The relationship between dreaming and strictly REM sleep has been one of controversy amongst researchers with varying hypotheses. Does REM-sleep physiology explain the dream experience, or is REM sleep neither a necessary nor a sufficient condition for dreaming to occur?14
One study hypothesized that different physiological processes underlie dreaming during REM and non-REM (NREM) sleep. Researchers concluded that although “dreaming” may occur during both REM and NREM periods, as previous researchers have suggested, the dreams obtained from these periods differ significantly in both quality and quantity and are likely to be produced by different processes.8