Science News had an interesting article about sleep patterns a few weeks ago (look here if interested). Recently, a few anthropologists and sleep researchers actually started looking at sleep patterns in other cultures. The 2 things I remember best from the article were:

1. People evolved with different wake/sleep cycles. This was a survival characteristic - in primitive times, if you had members of the tribe who were night owls and other members who were more alert in the mornings, etc., you had a better chance of the whole group surviving. People who nowadays are characterized as having "sleep disturbance" disorders are sometimes just folks whose natural body rhythms don't fit our industrial society's rigid 9-5 clock.

2. One primitive society from New Guinea had a set of tribal sleep patterns where the women (who did most of the child care and housekeeping, as with most primitive cultures) retired early, while the men stayed up late "performing rituals". My husband showed that to me, and said "See! I'm performing rituals!" I said "I thought you were just staying up late watching The Nashville Network." He said "Same thing..."

This doesn't explain why the best Irish sessions never seem to get going until after 11 PM (or later) - but maybe it's just part of performing the rituals!