A friend recently confided to me that he is feeling down, and has lost his appetite for life.

Like me, my friend is in his early 50s. So, after making a few sympathetic noises, I suggested that perhaps he is suffering from the mid-life slump.

In 2008, economists David Blanchflower and Andrew Oswald published a paper in which they presented evidence that psychological well-being is U-shaped throughout one's life.

According to their research, people generally enjoy their greatest life satisfaction during their youth and old age, and suffer a slump in happiness during middle age.

This seems to hold true for both men and women, and for everyone around the world.

After analysing their data, the economists concluded that "mental distress tends to reach a maximum in middle age".

This surge in mental distress, or dip in happiness, is what I meant when I spoke to my friend about the "mid-life slump".

The bottom of the curve

While thinking and reading about all of this, I came across an episode of the Hi-Phi Nation philosophy podcast entitled The Bottom Of The Curve, in which three people reflect upon their own personal middle-age slumps.

Interestingly, all three of them were highly successful people. Yet their successes and achievements did little to insulate them from the mental distress of middle age.

It wasn't that they looked back upon their lives and thought: "I wish I'd achieved more." Rather, they looked back on their lives and achievements and thought: "Is this it? Is there nothing more?"

The podcast's presenter summed it up like this: "If you don't achieve anything in the first half of your life, it makes sense that you're unhappy.

"But even if you've done as much as you can to achieve, you still end up feeling that something is missing."

The great Russian writer Leo Tolstoy (1828-1910) suffered a similar experience. By the time he reached his late 40s, he had achieved fame, wealth and international acclaim.

And yet, he sank into a deep depression, and was unable to see the point of it all.

He wrote: "My life came to a standstill. I could breathe, eat, drink and sleep, and I could not help doing these things, but there was no life, for there were no wishes the fulfilment of which I could consider reasonable.

"If I desired anything, I knew in advance that whether I satisfied my desire or not, nothing would come of it."

The same thing happened to the celebrated British philosopher and economist John Stuart Mill (1806-1873).

As a child, Mill was extraordinarily precocious. He began learning Greek at age three, had written a history of Rome by age six, and by 12 had acquired a graduate-level education. So, perhaps unsurprisingly, he arrived at his mid-life slump early too.

In his early 20s, he suffered what he described as a crisis in his mental history: a dark depression which left him deeply unsatisfied with everything he had ever achieved, or ever could achieve.

He wrote: "In this frame of mind, it occurred to me to put the question directly to myself: 'Suppose that all your objects in life were realised... would this be a great joy and happiness to you?'

"And an irrepressible self-consciousness distinctly answered, 'No!' At this my heart sank within me: the whole foundation on which my life was constructed fell down."

Perhaps my friend could draw some comfort from all of this. He is certainly not alone in suffering from the mid-life slump.

Furthermore, both Tolstoy and Mill navigated their way safely through their personal crises. And all three guests on Hi-Phi Nation have found ways to navigate theirs.

Best of all, according to Blanchflower and Oswald's research, my friend is likely to recover from his slump fairly soon, and will most likely be contented in old age.