Great ELI5, but I think you may have gotten those backwards? I’ve been trying to figure it out, but check this part of the paper:

“…with stimuli presented at diastole judged, on average, 9 ms longer (M = 305, SD = 25) than those at systole (M = 314, SD = 26).”

I think this saying in diastole (when the heart is beating), the time the participant felt an image was up was 9ms longer than when the heart wasn’t beating, and I would interpret that as time “expanding” since you feel like you’re spending a longer amount of time looking at a stimulus than you really are. Maybe this is just confusion about time “expanding” vs “contracting”.

Anyway, cool theory!

You know how sometimes a few minutes or seconds feel really slow, or they go by super fast? This paper focused on the small side, on those few seconds that sometimes feel slow or fast, and they thought “hey maybe it’s related to the heart beating.”

So they went a proved a relationship between when your heart beats and the perceived passage of time, specifically during the 200-400 millisecond windows of “your heart is pumping blood” and “your heart is chilling, waiting to pump blood”. In the first state, time seemed to expand (ie felt longer), in the second state, time seemed to contract (or felt faster) according to study participants.

They also found that this effect nearly completely went away as one becomes more aroused, likely due to other more important factors like your brain interpreting something more interesting, which could easily muddy the results.