Sleep helps us to retain the information that we have learned during the day. We know from animal experiments that new memories are reactivated during sleep. The brain replays previous experience while we sleep – and this replay strengthens memories overnight. Up to now, it was hard to demonstrate such a reactivation in humans, because the activity of individual neurons cannot be observed and most memories will activate entire networks of brain regions. Scientists have now applied new statistical pattern detection methods from the field of machine learning to get around these problems