If you've ever felt guilty for cancelling plans or declining invites after a day spent in front of the computer, you can rest assured that scientists know exactly how exhausted you feel.

While your office job may not require much of you physically, it undoubtedly requires a staggering amount of mental output, and this is why your couch is the most appealing place after nine hours sitting at a desk.

Your need to take a nap after work or disinterest in socialising comes down to something known as mental fatigue and actually has the same effect on the body's energy levels as a long run.

How validated do you feel right now, ladies?

Explaining the science behind it, director of sleep medicine at New York’s Lenox Hill Hospital, Dr. Steven Feinsilver, assured the public that their body's response to mental fatigue is legitimate, and feeling thoroughly exhausted after nine hours stressing at your desk is unsurprising.

'Your heart will pump and you’ll produce adrenaline whether somebody’s chasing you, or you’re just really upset about something,' Dr Feinsliver said of mental fatigue when speaking to The Cut.

Further to this, he explained that using our brain requires an inordinate amount of energy, but as we don't break a sweat like we would expending energy during exercise, we don't realise it.

'Your muscles normally aren’t sucking a lot of oxygen out of you,' he said.

'With exercise, they will. But the brain always takes a lot of your energy.'

'We’re animals that are designed to exercise physically as well as mentally,' he continued.

'Mental exertion is real, and certainly may be tiring, but I think it’s probably true that physical exertion makes it more likely that you’ll get a good night’s sleep.'

If you have a desk job and find yourself utterly exhausted at the end of every day, you need to ensure you stay hydrated and take short strolls around the office in order to maintain your wellbeing.