“Knowing your style in each decade is kind of like your career progression,” says Mr Olie Arnold, MR PORTER’s Style Director. “For most, your twenties should be playful and varied, thirties should be more refined, but still ambitious, and your forties should be concrete… but flexible. You should know what colours work for you and have a core wardrobe that has been tried and tested and is full of investment pieces that you trust.

“The flexible part comes in because your body shape may have changed,” Arnold says. “Your metabolism slows down, life changes (such as kids) mean that your free time evaporates or you’re now sitting behind a desk at work more than you ever have.”

This might not require rethinking your entire wardrobe. But this is a good moment to do a stock check and identify anything you’re missing.

“Buy less, but buy better,” Arnold says. “I rotate a small selection of product. If you treat and care for it as you’re supposed to, it will go the distance.”

“Good shoes, a classic watch, the best quality sweatshirt you can, a navy blazer – you start to do the cost-per-wear thing, and these will always win,” says Mr Johnny Davis, style director at Esquire UK.

“Men can get complacent as they get older and think that spending a lot of money on a good-quality item is enough,” says Ms Shakaila Forbes-Bell, fashion psychologist and author of Big Dress Energy. “Incorrect. You have to get things tailored. Not only will you look and feel better, but studies show that people are more receptive and think more highly of those wearing well-fitting clothes.”

“Alan Flusser, the author of Dressing The Man, says that style is 90 per cent fit and colour,” Davis says. “You just need to get that right.”

That old edict about jeans on men over 40? Put it in the bin. “I will never stop wearing denim,” Arnold says. “You may want to reconsider fit, not too skinny or too wide, but honestly that also depends on your figure. Denim is a must in my wardrobe.”

“You totally can wear jeans,” Davis says. “Though I did once ask one colleague why he always looked so well turned out and his answer was, ‘I just don’t wear jeans.’ So, if you want to appear ‘smarter’, then that’s a trick.

“Some men look amazing in jeans,” Davis adds. “The artist Takashi Murakami comes to mind for some reason. He’s got amazing style, loves a baggy pair of jeans.” Just avoid the bootcut look of Mr Jeremy Clarkson, Davis advises.

Whether you opt for jeans or not, Davis identifies the lower half as the area where what you wear can have the biggest impact. “It’s easy for men to forget their trousers,” he says, awkwardly. “And jackets and jazzy shirts get all the attention. But the opposite is cooler – an amazing pair of pleated trousers and a T-shirt looks great.”

“Clothes are a great leveller for telling you when you need to cut back on the pastries,” Arnold says. “Favourite pants a little tight all of a sudden? Maybe work is getting on top of you, you’re exercising less and comfort eating? If so, make some changes, as we all know that keeping in shape has greater benefits than just looking good. However, as we get older, most people’s shape does change, so go with it. Embrace it and size up.”

“Self-talk is a great way to address changes in your body in your wardrobe,” Forbes-Bell says. “Our go-to response, particularly when we’ve become more sedentary and may have gained weight, is to speak negatively about your body and claim you have nothing to wear.”

One answer to this is to reframe how you speak about yourself. “Psychologists Vanessa Patrick and Henrik Hagtvedt discovered that replacing the words ‘I can’t’ with the words ‘I don’t’ can positively impact goal-directed behaviour a sense of control,” Forbes-Bell says.

“So, instead of saying, ‘I can’t wear this because I hate how much my stomach has grown,’ you’ll say, ‘I don’t want to showcase this part of my body today, so instead, I’ll wear something that highlights a part I do like, like my arms’. This self-talk allows you to see the benefits of your wardrobe that you can be blinded to when you’re fuelled by self-loathing.”

Maybe it’s the increased awareness that time is finite, but as MR PORTER’s Senior Watch Editor Mr Chris Hall notes, the forties are the “business years” when it comes to investing in a watch. “It’s the period when men who haven’t yet done so feel that imperative to buy a proper grown-up one,” he says. “You want to avoid any insinuations of midlife-crisis shopping and buy something properly classic. It still needs to suit you, so that doesn’t mean reverting to a big dive watch because it feels like the right thing to do. The watch for your forties might be a Cartier, a Chopard or a Jaeger-LeCoultre.”

For experienced collectors, Hall argues that this is when you should look to consolidate and upgrade. (He argues the case for trading up and pulling the trigger on that long-term, Wish List timekeeper here.) “Put a couple of watches aside and focus on what you really enjoy,” he says. “That could be anything from a very serious chronograph from IWC to your first perpetual calendar. Or this might be when seasoned watch buyers graduate to rarer independent makers – you will start looking at Ressence and H. Moser with new-found interest.” The world is your Oyster Perpetual. Speaking of which…

“A great watch such as a Rolex will never be wrong and is the one item of clothing that will at least hold its value, if not increase,” Davis says.

“Just because you haven’t worn something in a long time doesn’t mean it’s not for you,” Forbes-Bell says. “It’s easy for us to lose ourselves. Often, we can find that version of ourselves that we once lost, in our wardrobes.”

By this stage of your life, you’ll also have a better gauge of which items have staying power. Use that nous to inform any subsequent purchases.

“As a father, I’m very conscious of leaving this incredible planet in as best shape as possible for the future,” Arnold says. “I’m a big believer in recycling product and buying second hand. And, when I do buy new, I make sure it’s as high a quality as my budget allows.”

“As per Tom Cruise’s walk-in wardrobe in Risky Business, there’s an idea that men – it’s always men – will have settled on their ‘look’ and they can simply reach for it every morning, like a uniform,” Davis says. “It really does seem to take a few decades to work that out. That might be, ‘I’m the suit guy’. Or it might be, ‘I’m the guy who only wears navy’.”

Think of this as finding your groove rather than being stuck in a rut, Davis says. “There’s liberation in being freed from the relentless cycle of fashion, of stepping out of it.”

“Deciding that looking good is no longer necessary is the biggest mistake,” Arnold says of surviving your forties. That doesn’t mean there’s a prescribed way that you should now dress. And don’t think you have to jump on every new trend going. Just stay engaged and keep experimenting. “Once upon a time, there would have been a list of sartorial no-nos,” Arnold adds. “But we live in a much more forgiving and freer time when it comes to expressing individuality, so the sky really is the limit.”

“No one cares what you wear,” Davis says, is the key lesson that his forties taught him. “Wear whatever makes you happy. No one is judging you. Unless you’re turning up to a funeral in a tracksuit, then really, get over yourself.”

The single best improvement that you can make to your life, and the best way to delay signs of ageing, is to get more sleep. Which is fine until children arrive – usually unwanted at 4.00am.

So, our top style tip for men in their forties? An eye mask. Not only will it help with the shut eye, you can use it to pull a reverse Homer Simpson (another dated reference) and pretend to be asleep. Stay still and maybe no one will bother you.