In US equity indices, most long-run return accrues during the night (close to next open), while the intraday session is nearly flat. A practical look at why this skew exists, traced to liquidity premium and supply-demand structure, with concrete rules, expectancy, and limits.
Split a single trading day in two.
One half is the night: the return you carry from yesterday's close to today's open, while the market is shut. The other half is the day: the return from open to close, while the market is live.
Stack each half separately over a long horizon, and something strange appears. In the major US equity indices, almost all of the long-run return accumulates at night, while the intraday session is nearly flat.
It defies intuition. We sit glued to the screen, trading, during the day. Yet over time, the bulk of the gain rides through the hours when we are doing nothing at all.
Harvesting that skew is the subject of this article: the Overnight Edge. The idea is plain: buy at the close, sell at the next open. Hold the night only, and sit in cash through the day.
Why should that carry a positive expectancy? The sources reduce to two. Compensation for holding risk through a thin, illiquid window (a risk premium), and the supply-demand plumbing of index futures and ETFs.
This is not a directional method. In the language of the taxonomy of strategies, it belongs to the family that harvests a structural anomaly. Not a chart shape, not an indicator cross. It is an attempt to harvest the time structure itself, the fact that return falls unevenly across the hours of a day.
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