"I wanted to explore the boundary between sleeping and waking: it's a fertile space and I wanted to see whether music could really send people into that space."

Richter rarely listens to his own work - except when it's one of his film scores, which include (2013) and (2012) - and he has never had a problem sleeping.

"The opposite really - I'll go to bed and literally 10 seconds later I'm asleep. You could say that I'm on the edge of sleep most of the time, and just waiting for the excuse."

In fact, the only thing that would get in the way would be if he had to listen to .

"I can't usually sleep if I'm listening to music. It seems to fire up my mind and I keep engaging with it to see what it's doing."

The last time he didn't sleep through the night was when his teenage children were babies "and I thought I can understand the criminally insane for the first time". Then there are nights when he has intended not to sleep.

The most recent example was the September premiere of , playing live on BBC Radio 3 in Britain through the night from midnight to 8am (making it the longest continuous piece the BBC has ever broadcast).

It was performed in the Wellcome Library - a splendid multistorey reading room in central London with huge windows and several floors on which a total of 20 camp beds were installed, with sleeping bags. The musicians sat facing east, looking towards the windows. "When the dawn came up, that was magnificent," Richter says.

There were seven musicians, including Richter on piano and keyboards, five string players and baroque specialist Grace Davidson as soprano. He chose a single female voice partly because he loves the genre ("Renaissance music is my heaven") and he wanted a pure Renaissance voice to create an ethereal sense of angels, but also because this is a lullaby, of sorts, and so should be sung with a mother's voice.

Eight hours is exhausting for musicians, and Richter wrote in breaks for everyone to take brief rests at different times, so they could stretch or make coffee or sleep (though Richter doesn't think that anybody actually did that: they were too wired).

On October 30, the Suho Memorial Paper Museum in Taipei also tested the recording with an outdoor audience in Daan Forest Park. According to label Universal Music, the recording has sold more than 100,000 copies (physical and online) globally since its September release.

The eight-hour piece has 31 sections - ranging from two minutes to 33 minutes - and titles such as , , , , , and .

Richter conceived of the work as a narrative curve, he explains, drawing the shape on a piece of paper. It is the shape of a story, with the protagonist going into the depths before emerging transformed. "Or a hammock," Richter says.

It ends with quiet, not with a jolt like an alarm clock. "I'm not a morning person," the German-born British composer says. "I didn't want to give anyone a shock."

might be the longest piece performed by the BBC, but it is not even close to the longest composition ever written, Richter says.

There's , which is a computer-generated musical composition created by Jem Finer to last 1,000 years. It started to play on December 31, 1999, and we are approaching the end of its 15th year.

"It's being performed in a lighthouse right now," says Richter, "and a John Cage piece that takes 639 years, which is being played right now in a church in Germany.

"It's a series of chords with individual tones and there's a funny calendar which tells you when the next note is going to be, which is in about 2020.

"People have written about as if it were some kind of record attempt, but I could just put repeat marks at the end and it would be 16 hours. It's not about that," he says.

Richter started composing when he was three years old.

Little melodies would come into his head, he recalls, which he would play around with until they worked.

"I thought everyone did it," says Richter. It was only a decade later, when he'd been playing piano for a few years and understood what notation was, that he began to write it down. But in his head he is still in that pre-notation world. He is always humming; his wife and three children joke about how much he hums.

"It's a liminal thing, humming," he says of the concept of being in-between or in transition. "And I'm always interested in liminal things."

Sleep too, is liminal. "It's a creative place - you could say it's where I do my best compositions."

Before he started, he wanted to know more about sleep and the brain so he turned to US neuroscientist David Eagleman, with whom he had collaborated a few years before. "David is a superhero: a neuroscientist by day and a novelist by night. He wrote this beautiful little book called [a play on the double meaning of the English for 'addition' and the Latin word for 'I am'] about what happens when we die.

"I wrote an opera - or I suppose 40 operas - for Covent Garden based on , and so it made sense that David was the first person I went to."

He found that research over the past few years has started to unpick the way the mind functions in sleep.

"I was able to incorporate the kinds of structure and grammar that foster a sleep state. That's not to say I was remote controlling anyone's sleep, but I was mindful of the kinds of things that might be helpful."

According to Eagleman, these helpful things are what you might expect, "very low frequency tones, repetitive tones, repeating structures".

"And I thought 'Great, that's what I do anyway'," says Richter. "And with I did get some comments along the lines of 'Max Richter's music sent me to sleep again, only this time he meant it to'."

If Richter wasn't a composer, he would have liked to have been an astronaut. "That was my number one job. Then when I was about eight I told my mother that that was what I was aiming for, and she laughed and said, 'But that's what everyone else wants to be'."

"Actually," he reflects, "I think that in a way, metaphorically, it's what I'm doing right now in my work. Every project I do is like a journey into the unknown."

is available on iTunes; a shorter edition is out on CD from Universal Music