Almost all of us have woken up from a vivid dream that seemed so real it sparked emotions of joy, peace, fear or just straight up terror. Those feelings are hard to shake, but it seems like the moment we try to share what actually happened in the dream, the specifics just seem to disappear.
Our forgetfulness is generally attributed to neurochemical conditions in the brain that occur during REM sleep, a phase of sleep characterized by rapid eye movements and dreaming.
But that might not be the full story. According to a new study in mice, published in Science, the REM sleep stage also contains a period of "active forgetting.” This most likely occurs to avoid information overload, the study suggests.
Interestingly, the neurons responsible for this forgetting are also the neurons that help control appetite.
Previous studies conducted focused on a hormone involved in regulating sleep in narcolepsy, the condition that makes people fall asleep unexpectedly. For this study, the researchers examined a group of neighboring neurons in the hippocampus. These produce the hormone (MCH), a molecule that helps regulate both sleep and appetite.
Scientists found through electrical recordings of sleep activity in mice and experiments involving neuronal tracing that these neurons also send inhibitory messages to the hippocampus.
This lines up with what scientists already understood about the hippocampus, which is that 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.
So, if the hippocampus is the last to go to sleep, it could very well be the last to wake up, according to reporting in Live Science. Theoretically, there is a window where you wake up with a dream in your short term memory, but as the hippocampus isn’t fully awake, the information isn’t moved to your long term memory bank and you lose the details of the dream.
In general, humans are very good at forgetting nonessentials. In fact, many of our thoughts, not just those we have while dreaming, are lost. We tend to recall only things that we think about often or that have emotional significance — a problem, a date, a meeting.
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