A neurologist has sparked widespread fascination online after sharing a simple 30-second test that he says can reveal how your brain is really functioning.

The test is deceptively basic and relies purely on instinct by closing your eyes and opening them when you think 30 seconds have passed.

According to the specialist behind it, how early - or late - you open your eyes can offer clues about alertness, anxiety, fatigue and mental processing speed.

The exercise was shared by Dr Ryan Worley, a Clinical Neuroscience and Brain Rehabilitation Specialist from the US, who posted the test to Instagram so followers could try it themselves, with wildly mixed results.

'How accurate is your brain's internal clock? This is a quick test that can give us insight into how your brain is functioning right now,' Dr Worley said.

'Your perception of time is influenced by things like: stress and anxiety (which can speed up your sense of time), fatigue, brain fog, or poor sleep (which can slow it down) and neurological function in areas like the cerebellum, basal ganglia, and prefrontal cortex.

'This is the kind of thing I use in clinic to get a better read on brain function. How your brain perceives time often reflects how it's processing the world around you.' 

In the video, viewed more than eight million times, Dr Worley explains that the goal is to estimate 30 seconds without mentally counting, instead trusting the brain's natural perception of time.

Dr Ryan Worley (pictured), a Clinical Neuroscience and Brain Rehabilitation Specialist from the US has revealed a simple 30-second test can offer clues about alertness, anxiety, fatigue and mental processing speed

After the first attempt, he suggests repeating the test while counting to see whether accuracy improves.

'By doing this with and without counting, you'll notice how much your brain relies on external structure (counting) versus internal regulation,' he said.

He says opening your eyes significantly too early may indicate a heightened state of alertness or anxiety, while opening them well after the 30-second mark could suggest slower processing linked to fatigue or distraction.

To avoid panic for those whose sense of time runs long, he reassures viewers that the timer continues for a full 60 seconds - meaning no one 'fails' the test.

The comment section quickly filled with people stunned by just how unreliable their inner clock turned out to be.

'Forcing myself not to count in my head was harder than any workout I've ever done,' one person admitted.

'Six seconds and I thought I was over the 30,' another confessed.

'I thought so many thoughts and ended up opening my eyes at 13 seconds,' a third wrote.

'32 seconds while not counting has honestly surprised me,' another said.