What percentage of the population would you say are *true* night owls?

Not just people who stay up a little later now and then, and would like to be, but people whose circadian rhythms actually cause them to be night owls and so they consistently live a nocturnal life. Just curious. And I percentage of the population of the world, not just one country or another, although, if you have any info on that, I won't be unhappy receiving it. 8)