Ever wondered what that gunk is that's stuck in the corner of your eye when you wake up?

A Stanford University neuroscientist and professor of ophthalmology has now taken to social media to reveal the revolting truth about the yellow crust.

According to Dr Andrew Huberman, who has 7.4 million followers on Instagram, the sticky substance is in fact a collection of dead bacteria. 

In a TikTok video clip that's been watched 1.5 million times, Dr Huberman explained that the gunk is a sign 'your eyes have successfully defeated bacteria in your sleep'.

'So when you wipe those away, you're taking the casualties of a war that you won during your night's sleep and whisking them away,' he said.

TikTok viewers have taken to the comment section to express their surprise at Dr Huberman's explanation.

'Bro, I thought I was crying in my sleep,' said one user, while another was thrilled to learn she'd 'been a winner in something my whole life and never knew'. 

Others thanked Dr Huberman for the reassurance that their eye mucus was normal.

'Damn, I thought I had bad eye hygiene becz [sic] I always have this, even when I wash them before bed,' wrote one viewer.   

But according to medical experts at The Cleveland Clinic, Dr Huberman's explanation doesn't tell the whole story.

While this discharge does contain some bacteria, it is also mixed up with mucus, tears and old skin cells.

During the day, this discharge is simply blinked away through tears. But at night, it naturally builds up at the edge of the eyes while they are shut.

However, only a small amount of eye mucus is healthy, experts warn.

An excessive amount could be a sign of infection, allergies, dry eyes or other issues such as underlying eye conditions.

Abnormal discharge is often more noticeable than normal 'sleep' in your eyes, Cleveland Medical Clinic states.

Common causes include conjunctivitis, an extremely contagious condition caused by infection or allergies, a stye and dry eye disease. 

Conjunctivitis, also known as pink eye, usually affects both eyes and makes them feel gritty, produce pus and itch. 

It happens when blood vessels in the membrane surrounding the eyeball become infected by bacteria or a virus and widen.  

Most viral conjunctivitis settles in two to three days, but some can last for two to three weeks.

The NHS-recommended treatment is simply a hot compress - soaking cotton pads in cooled, boiled water and gently wiping eyelashes to clean off crusts.