Right now, your brain is keeping track of the passage of time without your awareness, letting you focus on better things like reading this story.

This happens automatically, but not consistently. The brain's perception of time can fluctuate, with some moments seeming to stretch or shrink relative to each objective second.

While these wrinkles in time may be distortions of reality, technically they aren't all in your head. According to a new study, some originate in your heart.

Heartbeats set the pace for time perception, says senior author and Cornell University psychology professor Adam K. Anderson, illustrating the key role our hearts play in helping us keep track of time.

"Time is a dimension of the Universe and a core basis for our experience of self," Anderson says. "Our research shows that the moment-to-moment experience of time is synchronized with, and changes with, the length of a heartbeat."

These variations in time perception – or "temporal wrinkles" – are normal, researchers say, and may be adaptive. Previous research has also explored their origins, suggesting thoughts and emotions can distort our sense of time, making some moments seem to expand or contract.

In a study last year, for example, Anderson and his colleagues found that virtual-reality train rides seemed to last longer for passengers when the simulated trains were more crowded.

But many prior studies have focused on perception of relatively long time intervals, Anderson says, and therefore tend to reveal more about how people estimate time than how they experience it directly in the moment.

To shed more light on the latter, the new study looked for links between time perception and bodily rhythms, with a focus on natural fluctuations in heart rate. While the overall cadence of a heart sounds steady, each individual beat can be slightly shorter or longer than the one before.

Research has shown that heartbeats can influence our perception of external stimuli, and the heart has long been suspected of helping the brain keep time.

The researchers recruited 45 undergraduate students from Cornell to participate in the study, all aged between 18 and 21 years with normal auditory acuity and no history of heart disease.

They used electrocardiography (ECG) to monitor heart activity at a resolution of milliseconds, linking the ECG to a computer that would play brief tones triggered by the subject's heartbeats.

Each tone lasted only 80 to 180 milliseconds, and after hearing one, the subjects were asked to report whether they thought it lasted longer or shorter than other tones.

The results show temporal wrinkles at work, the researchers say. Subjects perceived tones to be longer when the tones were preceded by a shorter heartbeat, and reported tones as shorter when the tones followed a longer heartbeat.

"The heartbeat is a rhythm that our brain is using to give us our sense of time passing," Anderson says. "And that is not linear – it is constantly contracting and expanding."

While the heart may wield heavy influence on the brain's perception of time, it's a two-way street, the researchers note. Hearing a tone led subjects to focus their attention on the sound, an "orienting response" that in turn changed their heart rate and readjusted their experience of time.

Incorrectly perceiving the passage of time might sound like a bad thing, and sometimes it is. But while losing track of time can lead to trouble, there may also be adaptive benefits to the kind of temporal wrinkles identified in this study.

The heart seems to help the brain work more efficiently with limited resources, the researchers add, influencing how it experiences the passage of time on the smallest scales, and operating at time periods too brief for conscious thoughts or feelings.

"Even at these moment-to-moment intervals, our sense of time is fluctuating," Anderson says. "A pure influence of the heart, from beat to beat, helps create a sense of time."

The study was published in Psychophysiology.