"I had a dream last night and I woke up crying, but I just can't remember what I dreamed about!"
"I can only remember some fragments of my dreams, like someone said something, but I can't recall that 'someone' nor 'something."
Does this also happen to you? Have you ever wondered why we just forget many of our dreams? A new study carried out by researchers from Japan and U.S. showed that it might be our brains' active action.
Rapid eye movement sleep
Rapid eye movement (REM) sleep is the period when most of our dreams are made. The first REM period occurs about 90 minutes after falling asleep, when your eyes start to dart, your heart rates rising, your limbs paralyzed, and your brain waves awaken. You then start to dream.
Scientists have explored the role of sleep in storing memories for a time period longer than most of our ages. So far, it has been proved that sleep helps the brain store new memories, and REM is suggested to be the time when the brain actively eliminates or forgets excess information.
Recent studies in mice have also shown that the brain makes connections between neurons to enhance certain types of learning. However, no research had shown how this might happen until this study.
"This study provides the most direct evidence that REM sleep may play a role in how the brain decides which memories to store." Said Janet He, director of the program.
How REM influences memories
To understand the process, first you need to know about melanin concentrating hormone (MCH), a molecule that is involved in the control of both sleep and appetite. In an experiment in mice, a majority (52.8%) of hypothalamic MCH cells fired when mice underwent REM sleep whereas about 35% fired only when the mice were awake and about 12% fired at both times.
Many of the hypothalamic MCH cells send inhibitory messages to the hippocampus which is the brain's memory center. MCH cells are active during REM sleep, which means the cells may have a role in memory store of the brain.
Test of the idea
The researchers used lab mice with the genes of MCH neurons on and off. They examined the role that MCH cells played in retention, the period after learning something new but before the new knowledge is stored into long term memory.
The result showed that the mice’s memories were worsened during retention when the MCH cells were “on,” while turning the cells “off” improved memories. Further experiments suggested that MCH neurons exclusively played this role during REM sleep.
"These results suggest that MCH neurons help the brain actively forget new, possibly, unimportant information," said Dr. Thomas Kilduff, director of the Center for Neuroscience at SRI International.
"Since dreams are thought to primarily occur during REM sleep, the sleep stage when the MCH cells turn on, activation of these cells may prevent the content of a dream from being stored in the hippocampus—consequently, the dream is quickly forgotten."