A study published in Nature Neuroscience suggests that it's possible to dream outside of REM sleep cycles, the deepest and most restorative phases of sleep.
REM (Rapid Eye Movement) sleep cycles are the deepest phases of sleep, during which brain activity is also the most intense, compared to the wakeful state. That's why this sleep cycle is usually recognised as the time when dreams occur.
The body usually enters an REM sleep cycle around 90 minutes after falling asleep. It is a period of deep sleep that's particularly restorative. In adolescents especially, REM is the most important phase of sleep for learning, memory consolidation and social adaptation.
A single night's sleep contains an average of four to six REM cycles of around 90 to 100 minutes each -- it varies from person to person.
Researchers studied 32 volunteers who agreed to spend the night in a sleep lab wearing caps fitted with electrodes to monitor their sleep and brain activity. Each volunteer was woken at different times -- depending on the readings -- and was asked if they were dreaming and whether they could recall details about the dreams.
Previous studies have found that 80 to 90 percent of people can describe a dream if woken up during an REM sleep phase, compared to 50 to 75 percent in another sleep cycle during the night.
Similarly, the researchers found that many participants reported dreaming during non-REM sleep cycles but that they had more difficulty remembering their dreams.
A second experiment involving seven volunteers, who spent between five and 10 nights in the lab, found that participants dreamed 71 percent of the time during non-REM sleep and 95 percent during REM cycles.
The researchers also found a correlation between dreaming -- during both REM and non-REM sleep cycles -- and low-frequency brain waves occurring in an area in the back of the brain they called the "hot zone." The scientists were then able to predict when a person was dreaming by identifying a hot zone.