You saw your ex on the street last Monday, so why is it that you’re dreaming about him now? A day’s events often come back to us in dreams that night—but, just as often, they show up a week later. It’s the “dream-lag effect
”: During REM, the hippocampus takes five to seven days to transfer select memories to long-term storage in the neocortex, finds a study led by Mark Blagrove, director of the Sleep Lab at Swansea University. “Dream lag” is associated with more positive emotions about previous events—which scientists chalk up to the way we reprocess memories in our dreams. This means that if you spot your ex today, next week’s dream will put him in a softer, kinder light than tonight’s.