Why Can't We Remember Our Dreams?

You spend a third of your life asleep, a good chunk of which involves dreaming. But most often, you don't remember any of your dreams. And even on those lucky days when you wake up with a memory of the dream still floating in your mind, there's a good chance that in just a minute the memory will vanish into thin air and back to dreamland.

In waking life, such a case of quickly forgetting recent experiences would surely land you in a doctor's office. With dreams, however, forgetting is normal. Why?

"We have a tendency to immediately forget dreams, and it's likely that people who rarely report dreams are just forgetting them more easily," said Thomas Andrillon, a neuroscientist at Monash University in Melbourne, Australia. It might be hard to believe that you had a dream if you don't remember anything, but studies consistently show that even people who haven't recalled a single dream in decades or even their entire lifetime, do, in fact, recall them if they are awakened at the right moment, Andrillon said. [Can I Control My Dreams?] 

While the exact reason is not fully known, scientists have gained some insight into memory processes during sleep, leading to several ideas that may explain our peculiar forgetfulness.

You are awake, but is your hippocampus?

When we fall asleep, not all the brain's regions go offline at the same time, according to a 2011 study in the journal Neuron. Researchers have found one of the last regions to go to sleep is the hippocampus, a curved structure that sits inside each brain hemisphere and is critical for moving information from short-term memory into long-term memory.

If the hippocampus is the last to go to sleep, it could very well be the last to wake up, Andrillon said. "So, you could have this window where you wake up with a dream in your short-term memory, but since the hippocampus is not fully awake yet, your brain is not able to keep that memory," Andrillon told Live Science.

While this might explain why dream memories are so fleeting, it doesn't mean that your hippocampus has been inactive throughout the night. In fact, this region is quite active during sleep, and appears to be storing and caring for existing memories to consolidate them, instead of listening for incoming new experiences.

"Some data shows that [during some sleep stages] the hippocampus is sending information to the cortex, but not receiving any," Andrillon said. "This unidirectional communication would allow sending memories from the hippocampus to the brain's cortex for long-term storage, but new information wouldn't be registered by the hippocampus."

Upon awakening, the brain may need at least 2 minutes to jump-start its memory-encoding abilities. In a 2017 study published in the journal Frontiers in Human Neuroscience, researchers in France monitored sleep patterns in 18 people who reported remembering their dreams almost every day, and 18 others who rarely remembered their dreams. The team found that compared with low-dream recallers, high recallers woke up more frequently during the night. These middle-of-the-night awakenings lasted an average of 2 minutes for high recallers, whereas low-recallers' awakenings lasted for an average of 1 minute. [What Do Babies Dream About?]

Neurochemical soup

Our poor ability to encode new memories during sleep is also linked to changes in the levels of two neurotransmitters, acetylcholine and noradrenaline, which are especially important for retaining memories. When we fall asleep, acetylcholine and noradrenaline drop dramatically.

Then, something strange happens as we enter the rapid eye movement (REM) stage of sleep, where the most vivid dreams occur. In this stage, acetylcholine returns to wakefulness levels, but noradrenaline stays low.

Scientists have yet to work out this puzzle, but some suggest that this particular combination of neurotransmitters might be the reason we forget our dreams. The boost in acetylcholine puts the cortex in an aroused state similar to wakefulness, while low noradrenaline reduces our ability to recall our mental escapades during this time, according to a 2017 study in the journal Behavioral and Brain Sciences.

Sometimes your dreams are just not memorable

Do you remember what you were thinking about this morning when brushing your teeth? Our minds wander all the time, but we discard most of those thoughts as nonessential information. Dreams, especially mundane ones, may be just like daydreaming thoughts and deemed by the brain to be too useless to remember, the late dream researcher Ernest Hartmann, who was a professor of psychiatry at Tufts University School of Medicine, wrote in Scientific American.

But dreams that are more vivid, emotional and coherent seem to be better remembered — perhaps because they trigger more awakening, and their organized narrative makes them easier to store, Andrillon said.

If you are intent on improving your dream recall, there are a few tricks to try. Robert Stickgold, an associate professor of psychiatry at Harvard Medical School, suggests drinking water before bed, because it will make you wake up at night to use the bathroom. These "middle-of-the-night awakenings are frequently accompanied by dream recall," Stickgold told The New York Times.

Once in bed, repeatedly reminding yourself that you want to remember your dreams may increase your chances, and so does keeping a dream journal, some studies have suggested. Upon waking up, hang on to that fragile dream memory: Keep your eyes closed, stay still and replay the dream memory, until your hippocampus catches up and properly stores the memory.

Original article on Live Science.