New research helps explain why we forget our dreams.
When we sleep, our brains go through four stages. The initial three are non-rapid eye movement (non-REM) stages.
The first stage includes the transition from wakefulness to sleep, when the body slows down from its daytime rhythm and "twitches" its way into sleep.
The second stage, also of non-REM sleep, involves light sleep. The third stage of sleep is deeper, and it provides the profound kind of rest that one needs to feel refreshed in the morning.
Finally, the time when our brains do most of their dreaming is called the REM sleep stage. But why do we forget our dreams most of the time? And when does erasing the memory of our dreams occur?
New research in mice suggests that the REM sleep stage also contains a period of "active forgetting." This most likely occurs to avoid information overload, according to the new study, and the neurons responsible for this forgetting are also the neurons that help control appetite.
The new findings appear in the journal Science. 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.
Neurons that are key for sleep, appetite
Previous studies that Kilduff and Yamanaka had conducted together with their teams focused on a hormone involved in regulating sleep in narcolepsy — a condition that may cause a person to involuntarily fall asleep at inopportune times during the day.
The hormone bears the name orexin/hypocretin, and a loss of neurons that produce it in the hippocampus may be what triggers narcolepsy, Kilduff and Yamanaka have shown.
For their new study, the researchers set out to examine a group of neighboring neurons in the hippocampus. These produce melanin-concentrating hormone (MCH), a molecule that helps regulate both sleep and appetite.
The scientists already knew from previous research that these MCH-producing neurons would be active during REM sleep. But electrical recordings of sleep activity in mice and experiments involving neuronal tracing have revealed that these neurons also send inhibitory messages to the hippocampus.
Given that the hippocampus is key for learning and memory, the scientists wondered whether these neurons had "a say" in preserving memories.
"From previous studies done in other labs, we already knew that MCH cells were active during REM sleep," explains Kilduff. "After discovering this new circuit, we thought these cells might help the brain store memories."
Neurons that help the brain 'actively forget'
To find out, the scientists used genetic ablation in mice and found that "switching off" these neurons improved the rodents' memory.
Specifically, the researchers used standard memory tests that examined the rodents' ability to retain new information. That is, they tested the phase of memory retention, which occurs right after learning new information, but before the information becomes stored in long term memory.
During the memory retention phase, the mice with MCH-producing neurons switched off performed better in sniff tests.
Further memory tests showed that MCH-producing neurons only affected memory in this way when they had been altered during REM sleep. That is, the mice performed better at memory tests when the researchers had switched off the MCH-producing neurons during REM sleep. Switching off these neurons at any other stage of sleep or during wakefulness did not seem to affect the rodents' memory.
"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 explains.
"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."
Thomas Kilduff, Ph.D.