It’s 2am and dawn is beginning to break. Not outdoors – London is still dark – but inside the Royal Albert Hall, where, as the cleaning-crew’s hoovers rumble and bin bags rustle, the metallic gleam of first light stirs on the ocean.

Conjuring that light, sending it flickering up to the gallery in dancing arpeggios, is Anna Lapwood, a tiny speck of a figure across the hall at the Grand Organ, playing the first of Benjamin Britten’s Four Sea Interludes. It’s a strange scene, but for Lapwood it’s just another Monday night.

Since she was named associate artist of the Royal Albert Hall last year, the 27-year-old organist has become a star on TikTok. Lapwood regularly shares her overnight ­practice sessions – midnight to 6am is the only time this busy concert hall is available – with more than a million followers on social media.

“It’s a magical place at night,” she says. “Not scary at all. There are always people around, and they’re great at bringing me tea when they sense I’m flagging a bit. I did once have a nap in a corner when I was particularly tired, but I nearly scared a cleaner to death – they must have thought I was a corpse!”

Propping up her phone next to the organ console during each session, Lapwood captures chance encounters – from tech crews and security staff to Benedict Cumberbatch (“He was standing right next to me as I was playing; I was just thinking ‘Don’t cock it up!’”); requests shouted up at her (Bach, Dr Dre, the theme from Interstellar) – and employs technical tricks, if needed (who knew a pair of pencils wedged into the keyboard could help with a tricky bit of Hans Zimmer – holding keys down to sound bell chimes while Lapwood’s hands and feet are otherwise engaged).

One particularly memorable night-time encounter – with the British DJ and producer Bonobo’s crew – led to an unexpected cameo in the next night’s concert.

A spine-tingling video (below) of Lapwood setting a packed Albert Hall ablaze with enormous organ chords at the height of his set went viral, racking up more than five million views online.

“It was the best concert experience of my life. Everyone in the room was in this trancelike state together. I’m still getting ­people coming to my concerts because of that one gig.”

Radiating off the screen in every clip, however wonky the angle or rough the footage, is Lapwood’s energy: bright as her signature ­primary-coloured power-tailoring, relentlessly positive, curious, quick. It’s been a long day; she was walking in the Peak District this morning. But as we sit and talk in the middle of the night, Lapwood perched ­cross-legged on the organ bench, clutching a mug of tea, she’s alert, hyper-focused.

“I totally understand why some people struggle with organ concerts,” she says. “We spend so much time as musicians learning to ­communicate with an audience – how can you do that when your back is turned?” It’s not a rhetorical question; it never is with Lapwood, who comes ready-armed with solutions.

“Maybe we put up screens in the hall, talk about the music, get people up close to the instrument and make it come alive. There’s no such thing as a problem,” she mock-scolds. “There are just games we need to play to find answers.”

Which brings us back to social media. “If you can bring people right up to the organ console so they feel, even on a small screen, like they’re sitting here on the bench with you, then when they come to a concert they can understand and imagine what’s going on.” It’s an approach that’s working. Post-­performance meet-and-greets are increasingly filled with classical first-timers who’ve followed ­Lapwood from her film-music and day-in-the-life videos on the internet all the way into the concert hall.

This, she says, is why she does it. “You will always get put in boxes when you’re in the public eye. For a long time, I was the ‘Female Organist’ who spoke about gender all the time; now, I’m the ‘TikTok Organist’. I think people assume it’s a gimmick, that I’m not any good. You have to be strong in your conviction and self-belief, block out the noise. So much good can come from this platform, from being able to bring people along on this wild ride with this bizarre instrument. The positives far outweigh the negatives.”

A serious catalogue of recent achievements, including a solo record deal with Sony, concerto debuts with major orchestras and presenting gigs for the BBC (all ­slotted in alongside her “day job” as director of music at Pembroke ­College, Cambridge, where she founded a new girls’ choir in addition to the college ensemble), are enough to silence any doubters. But it’s not the only stereotype that ­Lapwood has in her sights.

“There are so many misconceptions about the organ,” she says, stroking the keyboard as she talks, as though it might be offended. “One is that it only plays loud, it’s bombastic, and that’s it. But it can be so delicate. You have such variety at your disposal: you can basically pretend to be any instrument in the orchestra, from an entire percussion section to the softest flutes and strings.”

The organ was the perfect fit for Lapwood, who spent “a lot of time in church” as a child. Born in High Wycombe to parents who met while singing in a choir (her mother worked in paediatric palliative care; her father was a vicar), there was “always music around” throughout her childhood.

While at school in Oxford, she played no fewer than 15 instruments (four of them to Grade 8 standard). She was always looking for the next challenge, the next sound. “Music was my most natural form of communication,” she explains. “I was quite shy – I still am. I had a bit of a stutter and didn’t always find it easy to communicate with people, but when I played, I felt I could say what I wanted to say.”

Her strangest instrument? “Probably the bugle. I found one in a charity shop, but I never really mastered it. Oh, and there was the electric guitar. I went through a week-long phase after watching School of Rock where I wore black eyeliner and played it very loudly.”

But it took Lapwood a while to fall in love with the organ as a teenager, wrestling with this hardest musical challenge yet. “I just couldn’t get my feet to work ­independently of my hands,” she explains. Typically, she persisted, and soon became the first female organ scholar in Magdalen College, Oxford’s nearly 600-year history. It was a steep learning curve.

“I think I cried every day of that first term. It was tough – I very nearly gave up the scholarship. But then I had what I call my Devil Wears Prada moment. I gave myself the exact chat that Stanley Tucci gives Anne Hathaway – ‘Are you really trying? Are you really putting your heart and soul into it?’ – and decided to give it one more term. I came back early after the vacation and practised for eight hours a day for the rest of the year, and by the end, I had totally fallen in love with it. Then I decided to make it my whole career.”

That career hits major new milestones this year. First, there’s a newly released EP, Midnight ­Sessions at the Royal Albert Hall. The repertoire is all film music: Lapwood’s own transcriptions of pieces from Interstellar, Pirates of the Caribbean and How to Train Your Dragon, all specially devised for the famous 9,999 pipes of the hall’s organ.

“When I was a teenager, my favourite thing to do was to sit at the piano and figure out how to play soundtracks, so this feels like coming full circle. It’s about taking music that’s familiar, that people love, and using it to make this slightly weird instrument feel more comfortable, more normal.”

A full album will follow in the autumn. “There’s more film music, some Debussy, Chopin, Philip Glass, lots of female composers: it’s me in an album, the first time I really feel that” – but first there’s a solo Late Night Prom on July 25. Lapwood finally gets to invite the world to join her after dark. It’s a typically eclectic mixture of music, all themed around the moon and stars. “I see it as a game,” she says of concert-programming. “Can you take Britten and put it right next to film music, or Philip Glass next to Bach, and make it work? It’s fun seeing how far you can push it.”

Does she worry at all about how some of her unorthodox programming choices will be perceived by others in the classical-music world? She grins, a TikTok-famous smile that could be seen from right across the hall.

“I used to be really frightened of not pleasing everyone,” she tells me. “I would read every comment, ­desperately try and do the right thing. Everything was so cautious, so careful, it was like being in a straitjacket. Figuring out who you are as a person while everyone is watching is a scary thing, but recently I’ve felt more liberated.

“I think the answer is to play the music you love. Everyone tries to tell you what is the right music and the wrong music, but, ultimately, if you play what you love, then that ­enthusiasm and joy and happiness come out in everything you’re doing – not just that piece.”

It’s nearly 3am, and there are still hours of practice to be done. As I head out of the Albert Hall, I hear the Grand Organ start up again. The rest of the world may be fast asleep, but Anna Lapwood is just hitting her stride.