Scientists are still not sure why we dream. But new research in mice suggests that the brain might be using periods of deep sleep to clear out residual memories to make room for fresh ones.

In a study published in the journal Science, researchers in Japan observed the mice's hypothalamus while the mice slept. During their deep REM sleep—which is associated with dreaming in humans—a type of neuron that produced a hormone called MCH had a sharp uptick in activity. The MCH neurons also appeared to be targeting neurons in the hippocampus, the brain region that consolidates memories.

In an experiment, researchers isolated MCH neurons in the mice's brains for observation. The mice were allowed to sniff and play with two toys, which were removed when the mice had become familiar with them. Later, the mice were given a familiar toy and a new toy. With their MCH neurons artificially activated, the mice sniffed them both—suggesting that their memory of them was worse. When the neurons were artificially deactivated, the mice were able to remember that they had already been exposed to the familiar plaything.

The ability of MCH neurons to go patrolling the hippocampus during REM sleep led the paper’s authors to suggest the brain might use this dream stage to do some neurological tidying up, getting rid of non-crucial information so fresh data can be processed.

Why would we want to forget things? An abundance of information can be overwhelming and inhibit the ability to make sense of new knowledge. But if something is truly important—a birthday, a PIN number, a vacation—the brain will hang on to it.

[h/t The New York Times]