Night Moves: Is the Overnight Drift the Grandmother of All Market Anomalies?

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When we first heard about the overnight effect – the propensity for stocks to deliver all their returns when the market is closed and no returns during the trading day – our first reaction was: that can’t be right! After some preliminary reading, our follow-on reaction was: who cares!? In talking with market participants and academics, we found that most people shared our reaction, if they knew about the effect at all. “Bad data,” “quack analysis,” “a statistical fluke” were the common refrains.

But after some more digging, we found plenty of evidence for the overnight effect and more vexing features to the puzzle. We found that not only did the effect exist at the index level as previously reported (see chart of S&P 500 returns below), but it also shows up in a suggestively clustered pattern in individual stocks returns, and is particularly strong in “Meme” stocks. Moreover, there are plenty of reasons to care about this market anomaly.