Sleep can be too much of a disconnection. We need to switch off, recuperate and regenerate ourselves for hours at a time, yet it feels like a bit of a gamble sometimes. If I go to sleep, will the world still be here when I awake? Will I still be here when I awake? What if I don't wake? Maybe better to keep going, shovel your mouth and brain full of artificial stimulants in order to maintain the constant bustle and agitation that 21st century lifeblood feeds on. You don't want to miss anything, do you? You can sleep when you're dead.

Finally craved for but never recognised once received, the world continues as our defences are lowered and furious ego-driven thoughts expelled. We close down while the drunks outside our windows howl or the clicks and whirs of the modern living space conspire against our efforts to disengage. Just trying to get to sleep can be a terrifying sensation, willing ourselves into unguarded limbo.

New Age music has long played a part in aid of blissful nodding-off, while ambient artists have dabbled in sleep research as much as sound-tracking everyday activity. Max Richter's near eight and a half hours opus goes way beyond anything previously conceived, and displays the same heightened ambition that enabled him to effortlessly restructure Vivaldi or dissect his European identity on 2002's recently revived Memoryhouse. He is a modern composer par excellence, without fear and seemingly at the peak of his powers.

The impact of Sleep, much as with the slow cinema of Ming-liang Tsai or Lav Diaz, comes from the sheer time spent with it and the amount of effort the audience allows itself to invest with the piece and open up to it. The first delicate piano chords of 'Dream 1 (Before The Wind Blows It All Away)' should ideally lull us into a cocoon of cotton wool gentleness. A hopefully slow unmooring softly into the night rather than any mounting anxiety from staring at the remorseless clock face, or paying rattled heed to our ever-chattering interiors.

As Sleep progresses, chopped up into 31 smoothly flowing segments, soft organ and string drones begin to emerge with occasional sorrowful soprano vocal from Grace Davidson. We may have bypassed any seas of tranquillity and are in a place filled with deep longing and sadness, Davidson's wordless lament being a voice of solace to accompany us now we are truly alone. Suspended in a state of constantly pulsing transformation, the music becomes like translucent vapour in the night around us as our consciousness hangs by the slenderest of threads.

While Sleep can be regarded as eight plus hours of ambient drift, it also grows into a piece of considerable emotional weight. It becomes a meditation upon our being alive, of our continuing to breathe as the world rumbles continuously in our absence. This is music that might best be experienced in a flickering semi-conscious state, those parlous and fraught seconds in which a gaping void is glimpsed in tiny flashes before cognition vanishes without warning to be replaced by emptiness. Although Richter created Sleep so as to soundtrack the ideal period of rest, any live performance of it will doubtlessly be a richly immersive and moving experience.

By Sleep's final stages, the circular piano and string movements have become heavier, weightier and somehow aqueous. They bob us along on a pea-green boat skimming the surface of a vast alien ocean. Alien, because sleep is really a weird concept; shutting down for a third of our existence. Richter is helping us to make sense of this time in which we lie prone and unmoving to the outside world while within the chambers of our subconscious we may be buffeted by uncontrollable turbulences and raging battles. The final section of 'Dream 0 (Til Break Of Day) bears witness to dawn approaching, but the darkness still clings like a fragile shroud coating the sleeper.