Brain cells work to actively forget memories during a specific phase of sleep. That's according to scientists, who hope their research could help to deepen people's understanding of Alzheimer's disease.

When you sleep, your brains clear out the memories you've made while you're awake, deciding what to keep and what to discard. Forgetting is an active process, explained the authors of the study published in the journal Science. But less is known about this process during the different phases of sleep, newsweek says.

The team found that during the rapid eye movement (REM) sleep phase—or active sleep when you are thought to dream—special nerve cells appear to actively contribute to forgetting. What are known as melanin-concentrating hormone-producing (MCH) neurons sit in the hypothalamus: a part of the brain which helps with a range of functions such as sleep, appetite, and emotions. Past research has suggested these cells help to control REM sleep patterns: activating the MCH neurons increasing time spent in this phase, while inhibiting them reducing transitions into this phase.

The team looked at MCH cells in mice, and found 52.8 percent were active during REM sleep, compared with 35 percent when the mice were awake.

They also turned MCH neurons on and off during memory tests. MCH cells appeared to send messages to the hippocampus to stop the formation of memories.