Snooze before tackling the to-do list

WASHINGTON U.-ST. LOUIS (US)—When it comes to following through on all those intentions, it’s best to think it over, then “sleep on it.”

People who sleep after processing and storing a memory carry out their intentions much better than people who try to execute their plan before getting to sleep, say psychologists at Washington University in St. Louis.

The researchers have shown that sleep enhances our ability to remember to do something in the future, a skill known as prospective memory.

Moreover, researchers studying the relationship between memory and sleep say that our ability to carry out our intentions is not so much a function of how firmly that intention has been embedded in our memories. Rather, the trigger that helps carry out our intentions is usually a place, situation, or circumstance—some context encountered the next day—that sparks the recall of an intended action.

These are the key findings from a study published online this month in Psychological Science. The focus of the work is on “prospective memory”—things we intend to do—as opposed to “retrospective memory”—things that have happened in the past.

Prospective memory includes such things as remembering to take a medication, buying a Mother’s Day card, or bringing home the ice cream for a birthday party.

While the vast majority of sleep literature in psychology is devoted to retrospective memory, this study is the first foray into the relationship between sleep and prospective memory, the kind of memory we put to work every day. The findings, researchers say, offer important contributions to the understanding of the role sleep plays in cognition as well as memory.

Let’s say that you intend to give a colleague a message tomorrow, says Mark McDaniel. Seeing the colleague the next day will be a strong cue for remembering to give the message. But, during the time your brain encoded the intention, you’re also vaguely thinking of a meeting the two of you will attend the next afternoon. The context of the conference room is weakly associated with your intention to give the message even though you haven’t really thought explicitly about associating the room with the message.

The study shows that sleep strengthens the weak association between the conference room (the context) and the delivery of the message (the intention). But sleep does little or nothing with the stronger association between the person and the message.

“We found that sleep benefits prospective memory by strengthening the weak associations in the brain, and that hasn’t been shown before,” says Michael Scullin, doctoral candidate in psychology.

“One of the more provocative findings we have is that sleep didn’t strengthen the link between the explicit cue, which is the person, and the intention, rather it strengthened the weak association and the intention,” McDaniel says.

The researchers believe the prospective memory process occurs during slow wave sleep—an early pattern in the sleep cycle—involving communication between the hippocampus and cortical regions. The hippocampus is very important in memory formation and reactivation and the cortical regions are keys to storing memories.

“We think that during slow wave sleep the hippocampus is reactivating these recently learned memories, taking them up, and placing them in long-term storage regions in the brain,” Scullin says. “The physiology of slow wave sleep seems very conducive to this kind of memory strengthening.”

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