New research on mice reveals how sleep makes the brain forget things

What a nuisance is a faulty memory. How many times have you forgotten where you parked the car? A few years ago, probably as a sign that my retirement was overdue, I spent literally half a day trying to find my car at a major New York airport. Fortunately, I am not alone. When people find out I am an expert on memory, the first thing they ask me is normally whether I can help them be less forgetful.

Indeed, excessive forgetting is a major problem, but “normal” forgetting is actually necessary. After all, it is more crucial to remember what is important right now than to remember everything. There’s no point in remembering the phone number of the house you lived in 10 years ago – that may in fact block your memory for your current phone number.

But exactly how the brain forgets unnecessary memories has long been unclear. Now a beautiful and rather exhaustive series of studies, just published in Science, offers a clue.

Research does indeed show that, in order to remember what is important, we need to forget what isn’t important. This can happen at two levels in the brain, a “cleaning” of irrelevant information as we retain and consolidate our memories, and a “blocking” of irrelevant information when we try to retrieve a memory. The positive effect on memory of blocking irrelevant information has been known since the 1950s.

The new study, which was carried out in mice, seems to finally reveal the secret mechanisms of forgetting during retention of memory. The authors claim that forgetting is due to the activation of specific “melatonin-concentrating hormone (MCH) neurons” located in the brain’s hypothalamus, which is involved in releasing hormones. We know that melatonin affects sleep – and MCH neurons are indeed involved in the shift between the two main sleep cycles: NREM to REM (REM sleep commonly associated with dreaming).