Why do most of the stock market's gains occur overnight? It has an overall loss during the daytime

External question — Personal Finance Stack Exchange Author: 7529 Original post: https://money.stackexchange.com/questions/125340 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. I saw a tweet from the Chief Investment Strategist of Charles Schwab that said the following: Since 1993 from @bespokeinvest : Buying SPY at open & selling at close every day (ie, only holding it during trading day): -13.9% return Buying SPY at close & selling at next open (ie, only holding it after-hours): +634.2% It included the following graphic: Why has the S&P 500 generated such a massive gain since 1993 during the overnight hours (4pm to 9:30am), and has an overall loss during normal trading hours (9:30am to 4pm)?