Dreams are remembered by some people more often than other people. The cause of this difference among humans has been a mystery for centuries - until now.  

The French National Institute of Health and Medical Research launched an investigation to find out why some people remember dreams long after they wake, while others report almost never recalling the experiences. Perrine Ruby, from the Lyon Neuroscience Research Center in France, led the investigation, which involved 41 participants. 

Ruby's group of investigators began the study, examining each type of subject. They found people who often remembered dreams had high activity levels in the temporo-parietal junction of their brains. This is the "information-proccessing hub, located toward the back of the brain in humans. The feature is found between the temporal and parietal lobes, giving the structure its name. 

"Dreaming is still poorly understood. Notably, its cerebral underpinning remains unclear. Neuropsychological studies have shown that lesions in the temporo-parietal junction (TPJ) and/or the white matter of the medial prefrontal cortex (MPFC) lead to the global cessation of dream reports, suggesting that these regions of the default mode network have key roles in the dreaming process," the researchers wrote in the article announcing the results, published in Nature. 

Ruby and her team believe this increased activity may cause dreamers to be more sensitive to external stimuli. This keeps them sleeping lightly while dreaming. This could help subjects remember the events more often, according to the dream researchers. 

"This may explain why high dream recallers are more reactive to environmental stimuli, awaken more during sleep, and thus better encode dreams in memory than low dream recallers. Indeed, the sleeping brain is not capable of memorizing new information; it needs to awaken to be able to do that," Ruby said.

Positron Emission Tomography (PET) was employed to measure brain activity in volunteers. While one group reported before the test they remember dreams only a couple times a month, the others recalled an average of 5.2 instances a night. Volunteers were measured when they were awake, and while asleep. Higher activity in this junction was observed during both day and night hours in "high dream recallers."

The number of dreams per night may also play a role in the study. Ruby believes subjects who recalled dreaming more often may have a greater number of the events per night than other people. 

In 2013, Ruby published a study which found people with higher levels of temporo-parietal junction activity were light sleepers. This study is the first to tie that finding to recollection of dreams.