Have you ever wondered how (or why) you forget some dreams and remember others? Heck, have you ever wondered why you remember any one thing while forgetting other nuggets of information, seemingly at random? Your passwords for important accounts and websites, a memory from your childhood, the details of a conversation you had last week, and even your weekend plans, and yes, dreams; all arbitrarily stick with you forever, for a day, a week, a month, or maybe just completely discarded from your memory.
A team of researchers in California and Japan believe they’ve uncovered an answer to this mystery, and it all has to do with a neuron called melanin-concentrating hormone (MCH), a molecule that actually helps regulate both sleep and appetite.
But first, let’s dive slightly into the processes and functions of sleep in relation to memory and even overall brain health. Sleep can affect both your mental and physical health, with better brain function, muscle recovery, hormone balance, longevity, and even fat burning reliant on your quality or quantity of rest. But at least approximately four out of every five people wrestle with occasional insomnia. The lack of sleep caused by insomnia can have a number of consequences to our health, while just going extended periods of time without sleeping at all can bring on immediate symptoms.
After about 19 hours without sleep, for example, your brain starts to function significantly slower. Your reaction time, attention, memory, and mental accuracy all drop significantly — the equivalent of having a blood alcohol concentration of 0.05 percent. Round it out to a full 24 hours without sleep and that brain function is on par with somebody who is legally drunk with a 0.08 BAC. Much of this is all because while you’re sleeping, your brain is busy cleaning away and flushing out cellular waste, prepping you for quicker mental processing and even a better mood. Without it, your brain doesn't get to perform these regulatory "housekeeping" functions.
The truth is, sleep doesn’t work like a bank account. You can make deposits to be taken out at your discretion later on, whether it’s day-by-day or throughout the week. In fact, interrupted sleep is actually worse than only getting a couple hours through the night.
“I think sleep is honestly one of the greatest biohacks and the biggest performance-enhancing thing that you can do,” says sleep expert and Oura Ring creator Harpreet Rai, who wasn’t involved with the recent study. “Sleep is the foundation of our health and the foundation of our body and mind.”
Your Memory Is Also Pretty Poor
Studies have shown that interrupted sleep makes it much harder to remember things as well as retain new information. The brain actually requires a continuous stretch of sleep for the things you learned and experienced the previous day to be retained.
Your Longterm Brain Health Is at Risk
Technically, your brain is still hard at work while you’re sleeping. While it’s simultaneously resting, it’s also busy clearing out proteins that aren’t meant to be left in your brain. Interrupted sleep doesn’t allow your brain to clean out or flush out those Amyloid proteins, letting them build up over time instead. The danger to this is actually pretty scary, as Amyloid proteins are linked to developing Alzheimer’s disease.
You’re Not Very Sharp
Think of a time you drove late at night while tired. Your reaction time and alertness were inevitably impaired. As mentioned, there are studies that show sleep deprivation can impair driving ability as heavily as alcohol.
Attention span and cognitive abilities are impaired just as heavily by interrupted sleep as getting very little sleep at all. Even your chosen sleeping position can impact brain health, with studies showing that sleeping on your back as opposed to sleeping on your side can speed up brain aging. Research once found that rats who slept on their back cleared significantly less brain waste than rats that slept on their side. Specifically, amyloid-beta, the protein fragment that’s a hallmark of Alzheimer’s Disease can be left in the brain, and this same effect can take place in humans who sleep on their back as well.
Now, with all that in mind, we can begin to wrap our minds around the recent findings of Thomas Kilduff, Ph.D., the director of the Center for Neuroscience at the SRI International research institute in Menlo Park, CA, led the research in collaboration with Akihiro Yamanaka, Ph.D., from Nagoya University, in Japan, who examined brain activity during the different sleep stages in mice. They actively “switched off” the MCH-producing neurons in some subjects so they could later test the memory retention of the mice through sniff tests.
They learned that MCH-producing neurons affect memory retention during REM sleep, the most restorative stage of the four sleep cycles. However, if the neurons were switched off during the other stages (non-REM), the rodents’ memory wasn’t affected. Therefore, the presence of MCH-producing neurons during REM actively works to make us (or help us) forget information we don’t need to retain.
"Our results suggest that the firing of a particular group of neurons during REM sleep controls whether the brain remembers new information after a good night's sleep," says Kilduff. "These results suggest that MCH neurons help the brain actively forget new, possibly, unimportant information.”
Kilduff went on to explain that “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.”
So the next time you get frustrated with yourself for forgetting a dream, cut your brain some slack. This is actually a sign that everything is working as it’s meant to, even if you don’t always get to pick and choose which dreams or memories stick around. The researchers argue this breakthrough is one step toward one day treating memory loss problems.
“This finding, if true and confirmed by other studies, represents a major breakthrough in understanding a fundamental memory mechanism,” writes Giuliana Mazzoni, Professor of Psychology, University of Hull and memory expert. “The methodology is rigorous and results convincing.”