It's almost midnight in Essaouira, a windy city on Morocco's west coast. It's summertime but the ocean breeze is strong, so people huddle in groups, hands in pockets, chins buried in their chests. We're on the fringes of a crowd of a few thousand, standing shoulder-to-shoulder, all facing a bright stage flanked by giant speaker stacks. Some people dance, others sway from side to side. A few stand still, eyes closed, minds elsewhere. Behind me, a group of teenagers in tracksuits crouch to roll a joint, looking over their shoulder for police. Women in hijabs wrestle prams through the crowd. There's a lean, gentle rhythm snaking from the speaker stacks, the sound of krakebs, a Moroccan type of castanet hand percussion. The krakebs entwine with the twack-ing sound of single, three-stringed bass instrument called a gimbri. There's no kick drum to follow, but the crowd submits to the rhythm's subtle charm. As the percussion loops, time stretches and minds drift.

We're listening to gnawa, a type of spiritual trance music found in Morocco and other parts of North Africa. This meditative sound has brought around 500,000 visitors to Essaouira for this year's addition of Gnaoua Festival, a celebration of this centuries-old style that takes over the city for one weekend each year. In addition to the official programme, which runs across several stages overlooking a harbour, musicians play to small groups gathered on street corners, while local vendors sell traditional clothing to sunburnt tourists. During the day, the atmosphere is happy and energetic. Thanks to the appeal of gnawa's endless loop, the population of the usually sleepy Essaouira has doubled.

The visitors are drawn to people like Simo Lagnawi, a musician who's spent his whole life around this ancient style. "Gnawa music chose me," he told me over the phone a few months later. "That's number one." A Moroccan with native Amazigh roots, Lagnawi now lives in London. He opened the city's first gnawa school and plays at festivals across Europe, introducing the style's hypnotic charm to audiences outside Morocco. He wears bright, traditional clothing and keeps his hair in locs that reach his shoulders. As he explained gnawa's qualities, I sensed him smiling on the other end of the line.

Traditional concerts in Morocco, called lila, often go all night, and can even last for days. The band, led by a gnawa master, or maalem, is tuned into the crowd, shifting the tempo according to their response. It's transportive music, a way for listeners to see inside themselves—and find God. "This is spiritual trance music for healing," Lagnawi said. "The rhythm you play depends on the people and how they feel. You have to join hands with the crowd, to help people get their anxieties out."

The hypnotic elements of gnawa also turn up in electronic music. It's usually the realm of producers who work with subtle, ultra-repetitive sounds that you might describe as loops. Most house and techno is made from loops, a term that commonly refers to a repeating section of music. In house and techno, most tracks comprise sections of four, 16, 32 or 64 bars, which means they must work within fairly rigid structures. But what happens within those structures varies wildly. On the one hand, we have tracks like, say, Todd Terje's "Inspector Norse," an upbeat disco-house tune that adds musical elements—squiggles, melodies, white noise—every few seconds, building to an explosion of bright melodies and tones, much like a pop song. Then there are tracks like Maurizio's "M5," a nine-minute dub techno track made up of little more than a bassline and percussion looping for what seems like an eternity. And though nothing much seems to happen, "M5" is among the best pieces of dance music ever made.

Some tracks loop for seconds before a significant shift, others loop for minutes. Some, like the avant-garde composer William Basinski's extended ambient experiments, can loop for hours. In many cases, the tracks we call "loops" aren't really loops, as the music is constantly shifting, if only slightly. But those subtle changes are what draw us into their spell. In the case of ultra-repetitive house and techno, these changes may be slight tweaks to the sound of the hi-hat or bassline—a touch of reverb here, a touch of delay there. A producer might toy with the rhythm pattern, moving a percussion hit slightly off grid, then back again. This creates a feeling of progression, even though there's not much happening at all.

Often just a groove decorated with bleeps and bloops, these loopy tracks are dance music at its most fundamental. Yet producers work magic with this simple approach. Where many artists and DJs target listeners with disruption (breakdowns, white noise), others make their case more quietly. It goes back to house and techno's early years, where pioneering artists in Chicago and Detroit crafted some of the most stunning dance music of all time, often made with only two pieces of equipment: a synthesiser and drum machine.

These influential US producers soon inspired artists in Europe, who embraced the minimalism central to early house and techno. They included the producers Moritz Von Oswald and Mark Ernestus, the producers behind Maurizio's "M5," who mixed Detroit's steely minimalism with the warmth of dub and reggae, creating a new style of music called dub techno under the name Basic Channel. In Frankfurt, the Chilean-German artist Ricardo Villalobos perfected the art of reduction, channeling a range of influences into a spellbinding brand of minimal house. A scene based around ultra-loopy, endless grooves (and endless partying) thrives in Romania, led by the DJs Rhadoo, Raresh and Petre Inspirescu, who shaped what became known as the "Romanian sound." And away from the dance floor, Basinski uses tape loops to craft moving ambient pieces that stretch for hours. A special emphasis on smooth, endless repetition unites all these styles. That, and the power to be transportive.

But why is it transportive? Put simply, humans love repetition in music. "Repetition serves as a handprint of human intent," Elizabeth Hellmuth Margulis, a researcher at Princeton University, once wrote. "A phrase that might have sounded arbitrary the first time might come to sound purposefully shaped and communicative the second." That applies to a pop song's chorus as much as the endlessly looping gnawa krakebs. It also might explain why, say, the simple bass notes on a dub techno track feel musical. The more repetitive a track, the greater its potential to hypnotise.

Margulis's research suggests that, as music repeats, listeners shift their attention across musical repetitions, focussing on different aspects of the sound with each new listen. For the first minute of Robert Hood's "Self Powered," you might focus on the bassline. But as it loops, your attention may shift to the hi-hats and the subtle ways they're manipulated. A great loop techno track hooks a lister with simple pattern. A listener who allows themselves to fall into the track actively follows the progression, whether they realise it or not. Subtle changes, like a slight filter effect on some bleeps, shifts the sound, bringing the listener even closer. The best producers, perhaps inexplicably, know exactly when and what to tweak, to the point where a simple series of tones and a four-on-the-floor kick becomes intensely powerful.

A lot of this music is labelled minimal, but that only tells part of the story. Almost all minimal dance music is loopy, but not all loopy house and techno is minimal. The work of pioneering house and techno artists makes that clear. Take K-HAND and Gerald Mitchell, producers who make rich, soulful tracks that don't build up or down but are incredibly impactful. K-HAND's "Somebody," for instance, is a steamrolling track engineered for peak time. For five minutes, the groove is constant and the arrangement is busy, with bleeps, piano keys and a synth drone weaving between the beats. Barely anything else happens, which means every subtle tweak—like when the melody briefly drops out—feels earned. "You want it to sound loopy but not too monotonous," K-HAND said. "You can run a four-on-the-floor kick for three minutes and call that a track, but people are going to wonder what's going on."

Artists in Detroit made repetition and careful manipulation of a single chord an artform, twisting abstract sounds into tracks that touched listeners all over the world. It's a long way from the skeletal, calculated sound that would come to define the European minimal that followed, where a clinical approach to production led to a colder, less human aesthetic. For Robert Hood, a producer who helped shape the early sound of minimal techno, the approach is far from clinical. "It's spiritual," he said. "You have to be instinctively in tune with your spirit and sound to know when it doesn't need anything else."

Talking over the phone from his home in Alabama, Hood spoke carefully but remained easygoing, getting more excited as the conversation progressed. We could have been analysing a new signing to his favourite sports team. He's spent the past 25 years working with music at its most reduced, so you might say he's spent most of his life searching for the perfect loop. Active since the early '90s, Hood is one of electronic music's key minimalists, having set an early benchmark for lean, reduced techno with his 1994 album Minimal Nation. A few months later, Internal Empire set another. Both are ultra-reduced, pairing snappy drum machine beats with bleeps, blops and squiggles of synth. Yet, like his Detroit peers, his sound is deeply emotional. It's music for the body and mind.

"When you hear an Aretha Franklin acapella, or Whitney Houston, you don't need anything else," Hood said. "That melody, the pureness and the tone in the voice, is enough. You thought you needed all these other elements for security, but when you strip all of that away, the clutter, then you find life. It's about destroying the box. I like to call it God's hammer."

Hood's instinctive approach to production feels at odds with his sound. His music is made with surgical precision, yet its emotional potential feels limitless. Elements are introduced one by one, each sound tweaked with the precision of high-end machinery. Yet Hood and many of the artists who make reduced dance music speak of letting go, or giving in to the machines. "Sometimes I'll sit for an hour-and-a-half and just let the loop tell me what to do next," he said. "Once it's moving my soul, I know it will move other people's souls. It's the spiritual connection I make between the drums, melodies and hi-hats."

Another pioneering Detroit artist, Gerald Mitchell, makes a similar point. He grew up attending black gospel churches in Detroit, where bands laid down extended, swinging grooves to sing along to. Like Hood, Mitchell is a loop master, proven across dozens stunning house and techno tracks. Dance music doesn't get classier than the psychedelic grooves of "Soulsaver" and "Multi-Dimensional Drama," which Mitchell released as The Deacon, or tracks like "Quetzal" and "Olmec My Brother" as Los Hermanos. These tracks, and plenty others, are little more than a groove with snatches of melody, rolling for upwards of five minutes without the energy shifting up or down. But the way Mitchell tweaks the chords, or manipulates the atmosphere, makes them transportive. It's house and techno stripped down to its essentials, yet it has the warmth of a full band.

Much of Mitchell's best work appeared on Underground Resistance, the shadowy Detroit collective that helped lay the foundation for techno in the US. And like Hood, who was also part of Underground Resistance, Mitchell recognises the power of a great loop. "First of all," he told me, "I'm a musician. One thing I learned from soul music, growing up in Detroit, was that there was always a basic groove. When I first heard techno, I didn't care for it because it was loopy. And it didn't go anywhere. And it was boring. But then I started to understand what the groove is about—that one groove, without moving, without the change."

It can also be about what rocks a party. DJ Deeon, a producer with roots in Chicago's '90s house scene, crafted one of the most distinctive styles out there, where X-rated vocals met looping drum machine grooves. Most of Deeon's tracks are relatively short, rarely over five minutes, with virtually no energy shifts up or down. This style became synonymous with Dance Mania, an era-defining label founded by Jesse Saunders in 1985. Dance Mania's beginnings were in more melodic, classic sounds, but it would eventually settle on the raw, ultra-loopy sound of Deeon and his Chicago peers. "I'm really just a bassline guy," he recently murmured down the phone from Chicago. "Not a melody type of guy."

Deeon's brand of house is raw (track titles include "Shake That Ass," "Suck It" and "Titties And Ass"), but there are hypnotic overtones. The tempo is quick but the groove is steady, tailor-made for the lean, stripped-back DJ style once prevalent in Chicago. "It's tracky and minimal," DJ Funk, a pioneering Chicago artist once said about his hometown's style. "You'll just have a beat, a bassline, a hi-hat, a clap, and a vocal sample." Deeon's tracks suited that type of use, where DJs used tracks like building blocks, chopping between tunes with fast cuts and plenty of energy. Yet, as with some techno, the kick drum's endless thump was entrancing. Deeon began producing with those parties in mind. "I had a TR-909 and a keyboard I would hook up to the drum machine," he said. "You could trigger the keyboard through the 909. It was easy for me, because it was like numbers, not like playing keys. It was a mathematic type of thing."

The sheer volume of music he has released—hundreds of tracks over more than two decades—reflects a loose, off-the-cuff approach to production. But that downplays the music's quality. Many of the tracks in Deeon's catalogue are timeless, reflected by the high-profile DJs still playing them. It's dance music at its simplest, but in the right DJ's hands, his tracks can be as transportive as Robert Hood's. Tunes like "Milkshake," a four-minute groove released in '95, sound utterly mind-bending in the right circumstance, its looping groove burrowing into your brain as the bassline twists and turns. The same goes for "Akceier 8" from 1998, little more than drums, a bassline and a twisted snatch of melody. "The build-ups in tracks nowadays can be awesome, but that's not my style," Deeon said. "Just get to the point and get out the way. I'm not going for a hit—basic is better."

Deeon might not spend 90 minutes listening to a single kick drum loop, but his early tracks were also produced in a state of total immersion. How else could you explain the artful way his basslines are manipulated, or the gentle tweaks to bleeps? Every element falls exactly into place. He makes the kind of loops you could listen to for hours. "Me and my little sister shared an apartment," Deeon said. "I would take the party speakers home, so I had a mini-system in my room. A real club system. You couldn't do anything but get into the mood when the bass is shaking the whole house."

A perfect loop for Deeon rattles his living room walls. For William Basinski, an ambient artist, it's something he lives in for days. Unlike the early US house and techno artists, Basinksi doesn't use synthesisers and drum machines to make music. Instead, he uses tape recorders to make tape loops, the term for sections of analogue tape spliced end-to-end to create endlessly looping soundscapes. It was a technique pioneered in the mid-20th century by minimal composers like Steve Reich and Terry Riley, who used tape loops to craft groundbreaking music. (One of those pieces is Reich's Come Out, which used tape to loop a four-second snippet of voice for 13 minutes. It remains a powerful document from the civil rights era.) Tape loops would eventually be utilised by Brian Eno on Music For Airports, the album that laid the foundation for modern ambient music.

Basinski also composed what's regarded as one of the greatest ambient works of all time, The Disintegration Loops, a series of four albums released across 2002 and 2003. Famously completed on the day of the September 11 attacks, Basinski's masterpiece totals five hours of music drawn from decaying tape loops he originally recorded in the early '80s. Understated but emotional, The Disintegration Loops move through a range of ambiguous moods, the sound of decades-old tape loops slowed down and stretched out. The source material of "Dlp 1.1," the first piece, is only a few seconds long. But Basinski stretches it for an hour, looping over, over and over. Sometimes fuzzy and sometimes clear, it's music to lose yourself in, an effect not far from the meditative trips producers like Robert Hood and Gerald Mitchell provide. Like them, Basinski's practice hinges on tapes and an endless search for perfect loops.

"I call them 'bubbles of eternity,'" he once said. "Sometimes an eternal perfection happens, where you can't tell the beginning from the end. It seems to create a timeless bubble that you can float in."

Speaking over Skype from Los Angeles, Basinski was friendly and energetic. The New Yorker has described him as more like a retired surfer than an avant-garde musician. He spoke about his deep, abstract loops the same way a big-wave rider might describe his favourite ocean break, using words like "holy" and "magical" to describe a successful studio session. Basinski's work has a mysterious allure, a quality he's chased for decades, long after his peers ditched tapes for synths. His is the kind of music you can listen to all day, either playing it in the background or staying intensely locked in. Your relationship with a piece may change over time. One day, it might be something calm for the office speakers. Another day, it could bring you to tears.

"The early '80s in New York was a prolific time for me making loops," Basinski said. "I made loops from the Muzak radio station, slowing them down. Sometimes something would happen—maybe I took it down two speeds—and I'd smoke a joint and leave it on for a couple of days."

Much of this music's power comes from the rudimentary methods and equipment it's made with. There are crackles in the recording, skips and hisses—the sound of analogue tape disintegrating. These "imperfections," as Basinski calls them, are vital, transforming the loops into something achingly human. There's charm in the way The Disintegration Loops break apart in real time. "Machines do things," Basinski said. "It's not like a digital loop, which is exactly the same thing over and over again. There's some element of nuance when you're using analogue tape. It can slip on the head, or maybe something will come in reversed—you never know what can happen."

The rugged charm of early drum machines and synthesisers also had plenty to do with the success of house and techno's first loop specialists. Robert Hood made some of minimal techno's greatest loops with a TR-909 drum machine and the Juno 2 and SH‑101 synthesisers. His mastery of these machines, using the drum machine to trigger the synthesisers, forever altered electronic music. An obsession with squeezing every bit of potential from his machines, down to making "a hi-hat sing," was central to the quality of his early music. In Chicago, DJ Deeon was doing the same thing, banging out jacking grooves on a drum machine he used to trigger notes on synthesiser. "Half-accident, half-intent," he said.

While DJ Deeon's house loops were rattling photos from the walls of the apartment he shared with his little sister, DJ Skull, another key Chicago artist, was sculpting a sound of his own. Moving between raw, abrasive techno and delicate, sensitive house, Skull landed on a lean, functional style characterised by slow buildups and rolling, looping grooves. He doesn't have the buzz of artists like Robert Hood or DJ Deeon, but remains a favourite of DJs who prefer their sounds lean and loopy. Many of his tracks reach upwards of ten minutes, usually looping the same snippet of melody over and over. Take "Get Em," a shimmering techno track he released in 1999 that artfully loops a set of chords for a stunning eight minutes. But by the time we reach the track's end, we've gone through passages of bright and dark, calm and chaos. "It goes back to the origins of jazz and early house," Skull said on the final day of a three-month stay in Berlin. "It was like a one-way street. There weren't big buildups."

Three decades later, Skull's music retains the loopy sound of the earliest house. The style and moods range from the abstract acid of 1993's "Don't Stop The Beat" to the sunny, lush melodies of 2016's "First Sight," but an obsession with repetition has been with Skull since youth. As a teenager, he would loop the melodic, instrumental sections of Giorgio Moroder records over and over, getting lost in the disco pioneer's signature arpeggiated synths. When he was able to gather a modest production setup, he tried to replicate that cosmic, ultra-loopy sound. And like Robert Hood, William Basinski and DJ Deeon, he was forced to squeeze every bit of potential out of the few bits of equipment he owned. "We didn't have much when we were younger," he said. "Just one drum machine and a synthesiser. You had to learn how to do all these things, how to tweak the hi-hats. You couldn't afford more equipment, but you could make it sound like there was more going on."

The '90s were a golden age for loopy house and techno, perhaps thanks to the relatively modest studios of its pioneers. There seems to be an inverse relationship between the prevalence of great loops and the amount of equipment available to producers. Where Robert Hood made the iconic Minimal Nation with a drum machine and two synthesisers, today's producers access a bottomless pit of digital synths and percussion libraries, drawing from dance music's entire history for sounds and samples. Perhaps there's some advantage to a modest production setup, one that encourages producers to get the very most from their machines. It could explain why, outside of veteran minimalists like Ricardo Villalobos, Thomas Melchior and Fumiya Tanaka, it's today rare to hear house loops on par with, say, Gerald Mitchell as The Deacon, tracks so perfect that it's impossible to imagine them being bettered.

Mitchell's best loops were made with machines, yet they feel distinctly human, a feeling heard across most of Detroit techno. That's not accidental. "As far as making a great loop, you have to put some soul into it," Mitchell said. "Even if you use a computer. Not everything has to align on the grid. I can be behind the beat, or a little ahead, or I might move the snare backwards. You need this fluctuation. That's the human side."

No loops are more human than the gnawa rhythm unfurling through the crowd in Essaouira. During that midnight concert, the band's clanking, shuffling hand percussion repeats over, over and over, as dancers call out the names of spirits over the gimbri strings' twack-twack-twack. The music is polyrhythmic and complex, each song blending into the next, guided by the gimbri's melody. The crowd is locked in, only breaking from their trance to applaud during the occasional moments of silence. "You're with the crowd," Simo Lagnawi, the gnawa musician, told me. "You put a big effort into connecting with them." The allure of a perfect loop was obvious that night in Essaouira, where thousands swayed to the krakeb, a gentle shuffle that felt like it would never end.