Do you want to know a secret? Of course you do. Come here. Bit closer. There is a top-secret formula for getting dressed which will make everyone think you have made a big effort and are on top of your game and full of brio, when in fact you know – but crucially only you know – that you are so comfortable you could lie down and take a nap without any need to get changed.

I was introduced to the concept of Secret Pyjamas by Alex, a contestant on The Great British Sewing Bee. The idea of secret pyjamas is to wear clothes which look polished and smart but are as comfortable as a pair of pyjamas. Secret pyjamas are completely different from tracksuits, which are pyjamas for young people, or pyjamas you wear to kid yourself you might go to the gym later.

The secret sauce in secret pyjamas is an elasticated waistband that only you know about. A waistband that doesn’t dig in is transformative in terms of how an outfit feels.

Silhouette is crucial here. You want elegance on the outside, comfort on the inside. For the secret part of the equation to work, your outfit needs to look polished and a little peppy. A long, baggy shirt over elasticated or drawstring trousers doesn’t look like secret pyjamas; it just looks like pyjamas. Don’t get me wrong, traditional pyjamas can be a very elegant look. A pair of silk PJs with heels and great earrings is a modern cocktail party classic.

But that’s way more effort than we are talking about here. You don’t want people to look at you and think you are wearing pyjamas, so you need to distract your audience by changing at least one element: it could be a comfy skirt instead of trousers, or the top half could be a waistcoat or blouse shape instead of a button-down shirt. The illusion tends to work best if the top isn’t too long: you will more easily fool the eye if the top half of your outfit only just sits over your waistband than if it hangs to your upper thigh.

You must – and I can’t stress this enough – resist the impulse to tuck your top half into your bottom half. There is a kneejerk instinct which many of us have – possibly embedded from school uniform days – to “smarten up” by shoving our top into our waistband. With secret pyjamas, this will ruin the illusion. Have the confidence to keep the two pieces free floating. The line will be smoother, that way. And because your top and bottom half can move independently without yanking on each other, you will be more comfortable.

A few clues on where to look: Reiss is excellent for good-value tailoring and trousers with the back half of the waistband elasticated, and a trompe l’oeil button fastening at the front which makes them look smart.

The Eden half-elasticated wide leg trousers (£128), come in an ivory shade which would make excellent secret pyjamas combined with its Carla knitted open-collar polo shirt (£148). & Other Stories is a great spot for longer skirts, many of which have elasticated waistbands and interesting tops to pair with them: I love its silk midi skirt (£125), in crushed olive silk,

Me + Em has been making secret pyjamas for ages. The retailer calls it “travel tailoring”. Its kick-flare trousers (£150), with a waistband that looks sleek but has plenty of give, look brilliant with its balloon-sleeve swing top, £125, which has an adjustable neckline (spot the hidden zip).

Even better is to mix and match what you have. If you’ve got a pair of drawstring trousers or a skirt with an elasticated waist, you are halfway there. So pull them out, then scan your tops for anything that could pass for a matching set – a waistcoat in the same colour, or a simple tank top, or a cardigan that you can wear buttoned up as a sweater. Voilà. Secret pyjamas. They are all around, once you know where to look. But let’s keep that between us.

Hair and makeup: Sophie Higginson using Hair by Sam McKnight and Kiehls. Styling assistants: Sam Deaman and Nyima Jobe. Model: Kitty Su at Milk. Top and skirt: Cos. Sunglasses: Bonnie Clyde. Bangle: YSSO