Marketsci on what overnight gaps tell us about the stock market

  1. The idea that the overnight market (close to open) doesn’t influence the daytime market (open to close) is usually correct, but NOT when the overnight market is moving violently like it has recently.
  2. Large gaps down exhibit a relatively strong negative correlation to subsequent daytime changes indicating a tendency to reverse some of the overnight gap in the daytime. I haven’t shown it in these statistics, but the larger the gap down, the higher the average return and the more negative the correlation (but also the higher the volatility) of the daytime reversal.
  3. Large gaps up do not exhibit a consistent influence in all market conditions. Also not reflected in these statistics is that as the size of the gap up increases, the correlation to the daytime market becomes more and more asymmetrical based on the broader trend. In up trending markets, these very large gaps up exhibit strong follow-through (positive correlation), but in down trending markets, very strong reversal (negative correlation).’