In support of this hypothesis, previous medical cases have found that when these same portions of the brain are damaged by stroke, patients lose the ability to remember their dreams, even though they can still achieve the REM (rapid eye movement) stage of sleep in which dreaming usually occurs.

The sleeping brain cannot store new information into long-term memory — for instance, if presented with new vocabulary words to learn while asleep, you will wake up completely unaware of what you heard. But this leaves open the question of how one is able to recall vivid nightly visions in the morning.

“If the sleeping brain is not able to memorize something, perhaps the brain has to awaken to encode dreams in memory,” said study author and neuroscientistPerrine Rubyof Inserm, a French biomedical and public health research institution. If awakened during a dream, the brain has the chance to transfer its faint flashes — via reiteration of the memory in one’s mind — into more long-term storage. This hypothesis has been dubbed the “arousal-retrieval model.”

“There’s a real question about the difference between dreaming, encoding memories of those dreams and being able to recall them,” said Harvard Medical School’s Robert Stickgold, a sleep researcher who was not involved in the study. “For someone to remember their dreams, all three of those things have to happen.”

Dream themselves exist first in working memory, or the memory we use to hold and manipulate thought fragments. Stickgold gives the example of hearing a five-digit number and then reciting it backward. But, like a fleeting dream, the series of numbers will erase in a flash if not put away into longer-term memory.

“Dreams are very fragile in short-term memory,” said Harvard Medical School psychologist Deirdre Barrett, who was also not involved in the study. She consults for a new mobile app, Shadow, that is aimed at improving users’ dream recall by waking them during REM sleep and having them dictate their dreams right away. “People do seem to form many short-term memories of dreams which, most nights for most people, are lost.”

In aprevious experiment,Ruby and her colleagues tested the arousal-retrieval model by measuring the sleep and wake cycles of a group of high- and low-recall dreamers. Using electroencephalography, or EEG, they found that the high-recall group had twice as much awake time throughout the night as compared with the low-recallers. Also, they found that the brains of high-recallers responded more strongly to auditory stimuli.

Upon seeing these distinctions between the two kinds of dreamers, Ruby wanted to suss out exactly which regions of the brain were behaving differently. Using positron emission tomography (PET) blood flow maps, they compared 21 male super-dreamers who consistently remember their dreams roughly five days a week with 20 low-recall males who could remember something only about two mornings per month.

They saw higher activation in the temporoparietal junction in high-recallers both during REM sleep and wakefulness, which could mean these people are more reactive to sounds or movements in the night and briefly awaken. Another part of the brain that showed higher activation in high-recall individuals is the medial prefrontal cortex, which has been found to be involved in self-referential thinking.

The study was published online Wednesday in Neuropsychopharmacology, a journal published by Nature Publishing Group.

Stickgold finds the study fascinating and convincing. As a 20-year veteran of dream research, he frequently has people asking him why they do not remember their dreams.

“Let me guess: You fall asleep quickly, never have trouble staying asleep, and you wake up with an alarm clock,” he said he tells them. “You never get a chance to remember!”

Kim is a freelance science journalist based in Philadelphia.

organic sulfur 2