Truro station at 10pm on a Sunday night is strangely alive. Platform three (of three) bustles with anticipation. As a train approaches from the west, bags are hitched on to shoulders and travellers, many of whom have an air of having done this hundreds of times before, gather at particular points on the platform.

The train squeaks to a halt, and doors are opened. This train will arrive at London Paddington at 5am; passengers will be ejected by 7am, and many of them will go to work.

Everyone is quickly on board. Beds are waiting, with crisp duvets, a few toiletries. The berths are narrow but comfortable. Doors slam and the train lurches back into motion. An attendant checks tickets and asks: "What would you like for breakfast?" The question stuns most travellers the first time they hear it, though the choices turn out not to be infinite. Having dealt with practicalities, travellers either go to bed or head for the bar, where many regulars meet up. The train stops at stations in Cornwall and Devon, then at Taunton and Reading, as it makes its way slowly to the capital.

The sleeper service that runs between London and Penzance is, in its current form, 30 years old this month. Officially known as the "Night Riviera" (a phrase that invites such scepticism that it demands quotation marks), it is less famous than the Caledonian sleeper that runs highland and lowland routes between London and Scotland. However, those who can afford a berth are prone to fall in love with the service the moment they step on board, and the numbers of people using it are going up at a rate of around 10% a year: nearly 30,000 people used the train last year.

The sleeper is much used by tourists during the summer months. Getting on a train in London, waking up and lifting the blind to peer blearily at the greenery around Lostwithiel (6.28am) is a wonderful way to start a holiday. However, the train has a less obvious role as a lifeline for Cornwall. The region has the UK's second weakest economy, and much of its employment is seasonal and low paid: Cornwall has a far higher than average rate of self-employment. The fact is, many people who live in Cornwall need to travel to where the work is and the sleeper allows them to cover the ground to the capital by night. This makes working hundreds of miles from home possible and keeps families anchored in Cornwall rather than being uprooted.

Paul Carroll, an IT consultant and married father of two, works in London and commutes by train from Falmouth, connecting to the main line at Truro. Like many, he travels flexibly. "I currently travel every other week," he says. He generally travels up on a Monday night, and comes home later in the week on either a night or a day train. "There are a number of regulars that I see," he says. "The train staff recognise me and some of them know my breakfast order. I like to use the sleeper going to London: I can get off at 6.30am at Paddington, grab a decent coffee, join the London buzz and arrive in the office at 7am."

Linda Evans is a seasoned night train commuter. "You do get to know people," she says. "And they become friends. It's all very civilised and a bit like a club. It's a great way to get to London." She recalls arriving at Paddington early one morning and finding the station utterly deserted because of heavy snow. "Everything was closed except Costa, and I heard an announcement that no tubes or buses were running." She managed to make her way to her office in Battersea all the same, and "no one who lived in London turned up all day. There was just me in the office."

I have used the sleeper often (last year a dash to Reading from an evening working in Portsmouth allowed me to connect on to the sleeper and get to my children's sports day). I love everything about the service (except its price tag) and the camaraderie I saw between regular commuters in the bar inspired me to use it as a setting for my new book, which is called, surprisingly, The Sleeper. In my fictional version, the main character, Lara, commutes between Cornwall and London by sleeper train, getting drawn into an affair with a fellow commuter and eventually vanishing from the train without a trace. Who wouldn't want to write scenes of train sex? I have since spent my sleeper trips trying to puzzle out the logistics: I know these things happen in real life, but the beds are very narrow. It would have to be a practical arrangement.

Daphne Du Maurier's Frenchman's Creek, set in the late 17th century, starts with Lady Dona St Columb fleeing London for the Helford estuary as fast as she can. The driver of her carriage remonstrates about the pace she insists on: "You forget that during the last two days we have covered nearly two hundred miles." For centuries, Cornwall was so far from London and the places beyond that covering the distance in a day was unthinkable.

It was the railway that opened up Cornwall to the rest of the country: this was crowned by the building of Brunel's spectacular bridge across the Tamar at Saltash, opened in 1859, symbolically connecting Cornwall to the rest of the country.

Cornwall's airport, Newquay, became relatively busy during the cheap-flights boom of the 90s and early 2000s. However, its fortunes have hugely declined. Passenger numbers are down by 25,000 in the past two years. Ryanair pulled out in protest at the airport's £5 environmental tax per passenger. Plymouth airport, meanwhile, closed in December 2011, contributing to Newquay's decline as the airports had shared an Air South West flight. Flybe, which operates a Newquay-Gatwick flight, has sold its Gatwick slots to easyJet. Meanwhile, the cost of fuel, added to the long journey time, makes driving unappealing for all but the most dedicated of motorists.

Paul Carroll reckons the sleeper train costs "about the same as flying or driving, especially when factoring in the cost of maintaining a car doing 25,000-plus miles per year". However, this is unfeasible for many workers. A sleeper berth, and use of the first class lounge at Paddington, costs a £50 supplement each way (£35 if you're prepared to share with a stranger). This brings the cost of a return ticket from one of the Cornish stations to London to around £200. Freelance workers needing to cover the ground quickly to get to a morning meeting factor it in as a tax-deductible occasional expense. Susannah Marriott, an editor, travels from Cornwall to London often for work, and says the sleeper is "a lifeline for freelancers who have moved out of London but don't want to let on for fear of losing jobs. I can arrive in Paddington at 5am, have a lie in on the train until 7am, have breakfast in the first class lounge, and still make it to a 9am meeting."

However, if you have to do it every time you go to work, it swiftly becomes impossible. The alternative is what many travellers affectionately term "cattle class": the chance to sit up all night in what First Great Western's website helpfully describes as a "Complimentary Seat (free)".

Mike Clark travels this way often, on his way to irregular work in and around the capital, and it is a completely different travelling experience. He describes the seats as feeling like "the first stubble on a shaved cat. They recline poorly and have a bend that restricts prolonged sleep. The staff have a look of knowing something you really don't want to find out. The toilet has a flush next to the emergency alarm, and the latter is often pulled, once by me."

While those tucked into bed happily sleep through the various stations and pauses along the route, those in chairs are woken at each stop by "a mighty distorted scrape of xylophone that rattles the brain", Clark says. And, he adds: "There is always a moment at about three in the morning when I have a minor freak-out. I am the only person awake in a confined room of gape-mouthed strangers in a rattling can. It looks like a zombie apocalypse."

These seats are where you find the student doctor commuting to a hospital placement; the teacher working in Reading while desperately seeking a job in Cornwall so she can be with her family all week; and the nurse going to where the work is available. Travellers at this end of the train often change into pyjamas and give a night's sleep their very best shot – but as Clark says: "I always arrive with jet lag, tired and achey."

However, even the shaved-cat seats are better than not being able to do the journey at all. As the sleeper's busy season gets under way, First Great Western assures its passengers: "We are proud of the sleeper's heritage and look forward to maintaining and improving services, keeping people moving in increasingly congested times."

"The sleeper is amazing," says Clark. "Daytimes are precious and night-times are not. And that is why I do it."

• This story was amended on 1 August 2013 to correct a reference to the Caledonian sleeper service.