False memory helps us think but we can't do it when we're tired

IT PAYS to have false memories. Our brains use them to generalise new information – but lack of sleep gets in the way. False memory was discovered in an experiment that asked volunteers to memorise lists of related words and then recall them. When they learned "bed", "drowsy" and "dream", about half later also remembered the word "sleep". That's because well-rested brains normally use "associative memory" to link related concepts together.