What would happen if stock markets were open 24 hours?
You would have trading teams who work shifts like any other 24-hour business areas. 

The power market is open 24-7; many companies have around-the-clock trading teams. A lot of analysis and other work that can be done in the daytime is still done in the daytime. 

A major source of income during nighttime - in the power market - is just being there and executing trades - when other people are not there. I think that is the main change, that liquidity would shrink and give more arbitrage opportunities.