Sleepless nights can increase your chances of forming false memories, according to researchers in Germany and Switzerland. But, as for so many aspects of life, it seems that coffee can save the day.

Although neuroscientists know that memories can be strengthened while we are asleep, it's been unclear whether false memories form as we slumber or whether they are only consolidated when we are asked to recall the information the following morning.

To find out, Susanne Diekelmann in Jan Born's lab at the University of Lübeck, Germany, and her colleagues asked volunteers to learn lists of words, each list relating to a particular topic. For example, they might learn the words 'white', 'dark', 'cat' and 'night' - all of which can be linked to the word 'black' - but black itself would not be part of the list.

The researchers then tested their subjects' memories after a night's sleep or a night spent awake. They showed them the list of words again, having added a few extra words, and asked them to recall whether the words had been in the original list. The sleep-deprived group gave more false responses than the group allowed to sleep. "A lot of subjects said, 'yes, these false words were presented before', and they were absolutely sure about it," says Diekelmann. "Sometimes they were even more convinced than on the real words."

Diekelmann suggests that it isn't sleep deprivation itself that causes the formation of false memories, but the act of retrieving them from storage. When the team kept one group of people awake for one night, let them catch up on their sleep the next night, and then tested them, the volunteers recalled the same number of false memories as those who hadn't been sleep-deprived at all. In the past "it has been difficult to separate fatigue effects from consolidation," says Brian McCabe, a memory and learning researcher at the University of Cambridge, UK. But this study seems to confirm that false memories are indeed consolidated at the moment of retrieval.

Diekelmann's team reported their results at the Federation of European Neuroscience Societies Forum in Geneva, Switzerland, on 13 July.

Coffee time

The scientists took the work one step further. If false memories were being generated at retrieval, they wondered, would a dose of caffeine reduce the effect of sleep deprivation? They took two more groups of volunteers, deprived them of sleep, and then gave them either caffeine or a placebo in the morning, one hour before their memories were tested.

The group given caffeine had 10% fewer false memories than those who did not receive any, an effect McCabe describes as "quite striking". The team suggests that this effect might occur because caffeine is known to affect the prefrontal cortex, a region of the brain that is impaired by sleep deprivation - and an area, says Diekelmann, that has previously been shown to help discriminate between things that have actually happened and things people have only thought about.

Diekelmann points out that understanding the false-memory process could be crucial to situations in which accurate recall is needed, such as when witnesses give statements in legal trials. McCabe agrees, but cautions that the work doesn't reveal whether the quality of sleep matters, or whether types of error other than false memories - for example, remembering a word correctly but in the wrong list - are any more likely.