What is time? Sorry, I don’t mean to be all existential with you — especially if you haven’t had your morning coffee yet. But chances are you’ve wondered more than once why, on some days, time seems to fly by in an instant, while, other times, it seems to drag on as you count every passing second.

A new study may offer some explanation. Researchers investigated how human brains perceive the passage of time, finding that there are time-sensitive neurons that can get worn out.

Worn out neurons

From sundials to atomic clocks that lose one second every 15 billion years, humans have always tried to objectively measure time as accurately as possible. Part of the reason why is because we’re not very good at tracking time as a group due to subjective biases.

While we have specialized sensory organs that can objectively determine stimuli such as vision or sounds, there is not one specific organ meant to measure and process the passage of time. It can only follow that time must be processed in the brain. This is why researchers affiliated with Osaka University in Japan and the University of California in Berkeley, California, decided to embark on a new study, in which they scanned the brains of 18 healthy volunteers while they engaged in various time comparison tasks.

First, the volunteers had to view a grey circle for a set length of time, 30 times in a row. They then had to estimate how long each instance of the circle shown on the screen lasted. This way, the researchers had an adaption period that they could use as a reference baseline.

Next, the volunteers were shown a test stimulus, whose duration they had to estimate. In situations where the adaptor duration was short, the participants tended to overestimate the time duration of the stimulus. When the adaptor duration was longer, the participants underestimated the passage of time.

For the duration of the experiment, the human participants were inside an fMRI machine that inferred brain activity from blood flow. The researchers noticed that neurons in the supramarginal gyrus (SMG) fired when images were flashed on the screen for specific lengths of time.

What was intriguing was that repeated exposure to a stimulus of a fixed duration caused neurons in the SMG to fatigue, whereas other neurons continued firing normally. When this happens, it seems like the humans’ subjective perception of time becomes skewed. The greater the neuron fatigue, the greater time distortion.

Although these investigations are still preliminary, the researchers believe there are some valuable lessons to be learned already. One is that the perception of time is heavily influenced by stimuli with a constant interval. Basically, repetitive tasks with fixed durations tire time-sensitive neurons in the brain, stretching or compressing how we perceive the passage of time.

It may be possible to one day stimulate these time-sensitive neurons in order to alter how we perceive time. Zapping these neurons with an electrical signal may help make time fly faster when you’re bored and engaged in mundane, repetitive tasks. Until then, don’t put too much faith in your brain’s ability to keep track of time. We’re pretty terrible at it.

The findings appeared in the journal JNeurosci.