Sleep on it!

If you want to learn something really well, the advice has always been to "sleep on it". But, according to University of Geneva  scientist Kinga Igloi, it looks like you also need to really value what you want to learn to get the most out of the effect, as he explained to Chris Smith...

Kinga - We knew sleep had an effect on memory consolidation but we did not know how reward and sleep interact, whether there is a selective effect of reward.  We might have thought of it because we generally remember rewarding things better, like for example, the great party you had when you were a child, but it had never been tested how this works in the brain.

Chris -   I suppose the evolutionary purpose of this is to prioritise the retention of information that in some way is likely to benefit an individual in the future?

Kinga - Yes, it is.  In evolution, we will try to remember things that are important, that we will need for our survival, or for a successful life.  So that is why we thought that rewards may help memory consolidation.

Chris - Tell us about the study design.  What did you actually do in order to probe that?

Kinga - So we make people come in early afternoon and learn pairs of pictures, preceded by a dollar coin or a cent coin, and we tell them that the dollar coin items will yield higher reward at the end of the study, so that they will be paid more if they remember the highly rewarded items better.

Chris - And where does the sleep come into it?

Kinga - So after they have learnt it, half of the participants take a nap and the other half stay awake.  During sleep, we monitor their sleep using EEG, which is electroencephalography.

Chris - And then having established that the person slept and what their brain activity is when they sleep or when they remain awake.  You then present them with those cards again and see how good they are at remembering the pairings you asked them to learn earlier.

Kinga - Yes, we show them the same pairs and so we test their memory.

Chris -   Right, well let's look at the sleep first.  Did the sleep improve retention of those memories?

Kinga - Yes, it did.  We had an overall affect of sleep, so sleep improved retention.

Chris - And then if you look at the value attached, was there a difference there too?

Kinga - There is a difference between subjects who slept and subjects who stayed awake straight after, and there is also a difference between highly rewarded items and lowly rewarded items. Subjects, remember highly rewarded items better.

Chris - And even better again if they sleep?

Kinga - Yes.  Then here we have to be clear, at the test point, so straight after sleep we do not have an interaction effect. So, at test there is only straight effect of sleep, so the subject who slept remember items better, and highly rewarded items are remembered better than lowly rewarded items.

Chris - Is this just a short term effect?  Is this something which is manifest only on that day you test them in the laboratory or if you pull people back days, weeks, months later, do you still see a preserved benefit through having had that sleep plus the high reward versus low reward situation?

Kinga - Yes, it is important to stress that we made people come back three months later, so a long time later, and we retested them on their memories and there was still an important effect of reward, and selectively for the group who slept

Chris -   Putting this together then, how do you think this is working and what did you see when you look at the brain activity?  Does that give you any clues as to what the sleep is doing to make this effect manifest?

Kinga - Yes.  What we find is that there is an effect of sleep and of reward, so that you remember highly rewarded items better and we have linked that in the brain to an activation of the hippocampus.  So we find higher activation in the hippocampus for highly rewarded items compared to lowly rewarded items for the subjects who slept versus compared to the subject who stayed awake.

Chris - So, if you want to learn something really well, you need to convince yourself that knowing it is extremely valuable to you and then sleep on it.