The US equity index futures opened for trading (as usual) at 5pm Chicago time on Sunday night. Two of the biggest US equity exchanges (NASDAQ and Arca) start tr

The US equity index futures opened for trading (as usual) at 5pm Chicago time on Sunday night. Two of the biggest US equity exchanges (NASDAQ and Arca) start trading in a pre-market session at 4am NYC time. The futures hit their own limit down band before the cash open, and I don't think the single-stock circuit breaker rules apply during the pre-market session for equities. There was a ton of volume in both the futures and the pre-market, but it was still dwarfed by all the activity at and just after the open. Most people just don't want to trade until other people are trading, so there's this sort of feedback loop that drives volume to the open.