“Time flies when you’re having fun” is an age-old saying that too often seems to be true. Have you ever been excited to go on vacation only to have it end too soon? By contrast, stressful situations seem to make time stand still. Studies have shown this phenomenon is due in part to our emotions, because the way we feel can impact the way we perceive time.

A team of neuroscientists in Lisbon, Portugal wanted to find a neurobiological explanation for why time is perceived in different ways. They sought to test their hypothesis that dopamine-releasing neurons may play a role in how we perceive time. Dopamine is a brain chemical, or neurotransmitter, commonly associated with the brain’s pleasure centers.

The team trained mice to estimate time. First, they played two tones, one shorter than 1.5 seconds, the other longer. The mice were trained to identify longer tones by placing their snout at a right port, and shorter tones by placing their snout at a left port. The mice were rewarded when they estimated the time correctly.

The next step involved measuring the activity of dopamine neurons. Results showed that when dopamine activity was high, the mice tended to underestimate the duration of the tone. Conversely, when dopamine signals were low, the mice overestimated tone duration. In other words, “Time flies when you’re having fun.”

In results published in the journal Science, the team noted the results might be able to be replicated in humans. They added that if these results do translate, it could have implications for treatment of disorders such as ADD.