This chart plots compounded return for SPY day and night (open-close, close-open) for the prior bull market of 3/03-10/07. Most of the period's gain occurred overnight, though day returns were strong in the initial rally. The overnight returns were consistently good over the period.

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This is a similar chart of compounded SPY day and night returns for the recent bull market 3/09-present. Unlike the prior one, the current bull market was up both day and night with advantage going to daytime returns.

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One possible explanation would be the visible hand of Ben and Co: heavily pulling levers during market hours to maximize return (of voters).

Source: www.dailyspeculations.com