This chart covers the overnight period per one bar for the ES. The open of the bar happens at 16:14:59 pm, and the close is at 09:29:59, just before the market open. I went back far enough to cover the huge drop we had at the end of last year. This is clearly not a quantitative analysis, but you can easily see how many red vs. green bars there are. Obviously, if you bought the close and the bar is red, then the market opened even lower, and hence you would have lost money. Even during the recovery since Xmas day, there are still many overnight sessions which would have lost you money.