nitrogen narcosis is also called “l’ivresse des grandes profondeurs” or "rapture of the deep". nitrogen comprises 79% of the air, but at surface pressures it has no sedating effect. at greater depths, however, nitrogen affects the brain in the same way as nitrous oxide (also known as laughing gas) and other anaesthetic gases. the effect is similar to the effects of alcohol, and to some extent there is cross-tolerance. unlike alcohol, the onset and disappearance are near instantaneous. a diver may be quite clear-headed at 20 meters, and yet giddy and silly at 30 meters. ascending to 20 meters will almost instantly clear the head.