Why we forget some dreams, and other cool brain science

Ever had vivid dreams all night that you can’t recall the next morning? You’re not alone. Research shows that the brain actively and deliberately deletes these dreams until there’s barely a trace left of them.

What’s more, it seems that the brain cells (neurons) that make us crave yummy foods also help us forget crummy memories.

Representational image. Image by Wokandapix from Pixabay

Read on to know why and how the brain does that.

Stages of sleep

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Researchers have been trying to unpack the puzzle we call sleep for centuries. Here’s what we know: one, sleep is essential for survival. All animals and birds sleep and have continued to sleep regularly throughout evolution. Two, there are four stages of sleep. The first three stages comprise non-rapid eye movement (non-REM) sleep. The fourth, REM sleep, is when we dream.

• Stage 1: When we feel drowsy or try to sleep, this marks a shift from wakefulness to sleep. Our heartbeat and breathing slow down, and our muscles start to relax.
• Stage 2: This is a period of light sleep when our muscles relax more and the heartbeat and breathing slow down further.
• Stage 3: It is marked by deep sleep; the kind of sleep that makes us feel refreshed and energized in the morning. During this calming period, our heart rate and breathing touch the lowest point and muscle relaxation peaks.
• Stage 4: In-between non-REM sleep, we experience short periods of REM sleep (stage 4). We dream during REM sleep. Typically, the first time we fall into REM sleep during the night is 90 minutes after dozing off. During this period, our eyes make rapid movements from side to side even as the eyelids remain closed. Our breathing and heart rate increase, the muscles of our hands and legs become temporarily paralysed - this prevents us from acting out the dream.

What’s new

The new research by a combined team of Japanese and American scientists shows that REM sleep — when we dream — is also a period of active forgetting. Further, the researchers found that the neurons that induce appetite also help us to forget things the brain deems too unimportant to remember.

The research team led by Shuntaro Iwaza at Department of Neuroscience II, Research Institute of Environmental Medicine, Nagoya University, Japan, published the results of their study in Science - a well-regarded peer-reviewed journal.

“Forgetting during sleep may be controlled by neurons found deep inside the brain that was previously known for making an appetite-stimulating hormone,” Izawa, et al., wrote in “REM sleep–active MCH neurons are involved in forgetting hippocampus-dependent memories,” published on 20 September 2019.

Shoulders of giants

To be sure, this is not the first study to look into the role of sleep in making new memories. Francis Crick — one of the scientists who won the Nobel Prize for discovering the double-helix structure of the DNA molecule — pointed out that REM sleep also filters out excess information or memory. Forgetting dreams is possibly a part of eliminating excess memories to prevent memory overload.

Thomas Kilduff, a senior researcher in the new study, pointed out that previous studies had also established the link between sleep and melanin-concentrating hormone (MCH) which regulates appetite and is also active during REM sleep.

What Izawa et al. were interested to know now was whether MCH was causing the hypothalamus - a part of the brain involved in crucial body functions like sleep, hunger, thirst, and memory formation - to actively forget some things. The answer, they found, is a resounding yes.

Janet He, programme director at the National Institute of Neurological Disorders and Stroke (NINDS), which funded the study, explained that this finding may have implications for treatment for post-traumatic stress disorder and Alzheimer's Disease in the future - patients often forget things in both conditions.

“This study provides the most direct evidence that REM sleep may play a role in how the brain decides which memories to store,” Janet He added.

Health articles in Firstpost are written by myUpchar.com, India’s first and biggest resource for verified medical information. At myUpchar, researchers and journalists work with doctors to bring you information on all things health. For more information, please read our in-depth article on Sleep Disorders.