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It is often said that the best way to get to the bottom of a hard problem is to sleep on it. But how accurate is this advice?

A new study forthcoming in the journal Psychological Science may have the answer.

A team of researchers led by Kristin Sanders of Northwestern University designed an experiment to test whether cueing music associated with unsolved brain teasers during sleep increased people’s ability to solve those same brain teasers the next morning. Fascinatingly, they found that it did.

“Numerous studies in humans and other animals demonstrate better after sleep compared with a similar time awake,” state the researchers. “Sleep-related memory processes not only strengthen but also can optimize, organize, and transform information.”

To arrive at this conclusion, the researchers invited 57 students at Northwestern University to participate in a three-day, two-night study. On the first day, participants were presented with a series of difficult spatial and verbal puzzles. Here is one example. (The answer is provided at the end of this article.)

On a wall outside a closet door are three standard on/off switches. One (and only one) controls a light bulb inside the light-tight, well-insulated closet. The other two switches do nothing. You can only open the closet door once, and cannot change any switches after the door is open (or re-closed, for that matter). Damaging or disassembling the door, walls, or switches is against the rules. Within these constraints, how can you determine with certainty which switch controls the light bulb? —Sanders et al. (2019)

Forty-two puzzles like the one presented above were used in the experiment. Importantly, each puzzle was paired with a sound clip. The experimenters encouraged participants to try to solve the puzzles as well as to remember the different sound clips associated with each puzzle. Participants worked through the brain teasers until a total of six puzzles remained unsolved.

Next, participants took home a sleep monitoring device and sound cueing system that gathered data and played music while they slept. For instance, every time the machine detected slow-wave sleep (i.e., the type of sleep in which and memory re-organization is thought to occur), the machine cued music associated with some, but not all, of the unsolved brain teasers.

The next morning, participants were asked to try to figure out the puzzles they were not able to solve the day before. The experiment was repeated later that day and night, and participants again attempted to solve the unsolved puzzles the next morning.

Over the course of the two experimental sessions, the researchers found that participants were 55 percent more likely to solve puzzles on their second try when the corresponding sound clip was surreptitiously played while they slept. They also reported that the effect was not dependent on the type of puzzle; solve rates for both spatial and verbal puzzles increased when cued during sleep.

The authors conclude, “By demonstrating for the first time that targeted reactivation of puzzle memories during sleep improves problem-solving the following morning, we add to a growing literature recognizing sleep as useful for both strengthening and reorganizing memory.”

So, the next time you're faced with a difficult problem, sleep on it...and play some music to remind yourself of the challenge.

Brain Teaser Answer: Turn on switch 1 and leave it on for a couple of minutes, then turn it off; turn on switch 2 and open the door. If the light bulb is on, switch 2 controls it, if it is off and warm, switch 1 controls it, and if it is off and cold, switch 3 controls it.​​​​​​

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