The Lying Down Concert: Earthrise, Royal Opera House

We should lie down to listen to music much more often. Gravity pulls away the thought and frown lines, smoothes the intellectual tracks and folds on the face, while you feel the blood in your head pumping lushly to dreamier parts of your brain. Joanna MacGregor’s If-A-Tree festival at the Royal Opera House this weekend may well be hitting some fey bases along its way, but Earthrise: The Lying Down Concert - was a spectacularly enjoyable opening event.

The Floral Hall became, aurally and visually, something more of an Arboricultural Hall, dark-lit, a black carpet throughout on which hundreds of our bodies lay under a cumbersome sculpture suggesting blasted fir trees hanging from the wonderfully weightless barrel roof of Victorian engineering that is this treasure of a building.

A sonic atmosphere created by Scanner with electronically manipulated forest noises, and a sophisticated lightscape of trickles, dancing dots, mists and rain, gave a surreal electronic experience of natural noise and light that was more interesting if taken for their artificial skills than for any illusory transportation to the natural forest. Manmade intervention everywhere, pretending to be nature (but then so was Capability Brown’s landscape gardening three centuries ago).

But even if the sculpture was pretty hideous (called Floating Forest, it dangles rather than floats, its black mesh sails stuck on like black Post-It notes at the top of spiky stripped trunks you can imagine freshly harvested in forest clearance), from a lying-down position you could gaze past it into the glass curves of the Floral Hall ceiling and to the darkening sky beyond, or you could turn your head and look through the end window upon the rustling trees outside - and the translation worked.

Created by a committee to fulfil a PR brief - the Deloitte Ignite annual weekend is a social-engineering device at the start of the Royal Opera House season to attempt to lure in the holy grail of a New Audience - this could well have been one horrible goulash. By benign chance, what resulted (at least in my head) was an atmosphere in which many different kinds of music could co-exist, like unrelated species of trees or animals. Electronica, live piano, a flute, a huge choir, Messiaen, Tallis, a trapeze aerialist. A jumble in any other setting, but magically cohering in this somehow enabling and uncritical space. Best not to scan the programme notes beforehand - just to be travelling by your ears and senses on McGregor’s conveyor belt from forest noises to a piano playing Takemitsu, to a near silence out of which steals the sound of a woody Japanese flute, and then to be taken totally by surprise by a massed choir.

The title of the evening was taken from a large-scale new choral work by Alec Roth, inspired by the Earthrise photograph of the earth brought back from space by the Apollo 8 crew, and sung by its dedicatees, the Ex Cathedra choir, from a breathtakingly spotlit setting on the shadowy balcony overhead. Its three substantial movements in a stately modern interpretation of 40-part medieval motet-singing were richly interlaced with other sights and sounds: some of Messiaen’s pianist birdcalls, Petites Esquisses, played exquisitely indeed by MacGregor (her fingers are so alive that even through speakers, meshed with the electronica, they instantly sound live and you lift your head to look for the pianist), traditional Japanese flute strains from Kiku Day on the shakuhachi, and the climactic finish of Thomas Tallis’s heartstopping 40-part motet, Spem in alium, evidently Roth's inspiration. And three times a slender aerialist, Ilona Jäntti, quietly ascended a white skein of silk to play with unbelievable weightlessness in its heights, furling the silk around her like Thumbelina dancing inside a lily.

Now, possibly there were diminishing returns in the aerialism outings, possibly a certain sameyness in some of the flute music, and yes, I did think the sculpture was an eyesore (I imagined all of this happening in a lush botanical greenhouse like Edinburgh’s or the Eden Project, or a butterfly house among real tropical trees and vegetation). But lying down and looking up feels different, healing, enriching and invigorating, a way to surrender and yield to what you’re experiencing, sensing the inside and outside worlds simultaneously, rather than to home in, analyse and judge. All the performers should be unstintingly congratulated, but the format’s a winner of itself - it should be given regularly around the country, on the NHS, to contribute to reducing health costs, and surely the next step has to be for lying-down Proms.

• See what's on at the Royal Opera in 2010-11