Summary. Some SPX sessions hold a narrow range and reverse every extension; others trend from the open. The sign and shape of dealer gamma is one of the better-documented reasons for the difference. This guide follows a session from the pre-market map to the close, explains why gamma regimes persist and when they tend to change, and sets out how the levels combine. It describes tendencies, not rules.
1. The overnight setup
Before the cash open, the useful questions concern the map for the coming session. Where is the gamma flip relative to the level implied by futures? How far away are the call wall and the put wall? How much of the gamma near spot sits in the same-day expiry?
With spot below the flip, dealers are estimated to be net short gamma, and their hedging adds to moves in either direction, not only higher. A rally that reaches the flip enters the long-gamma regime, where hedging starts to lean against further gains, so the flip can be the first real test overhead. A call wall just above spot means a rally meets heavy hedging supply there. Large same-day put positions below spot, held short by dealers, mean a decline into those strikes can accelerate rather than stall.
2. The cash open and the first hour
Dealers hedge overnight in SPX futures, and SPX options trade in Cboe's extended hours, so the book is not left unhedged while the cash market is closed. But overnight liquidity is thin, and the cash open is when the whole chain reprices against a live index and most participants, not only dealers, rebalance. That makes the open the noisiest part of the day.
The regime still shows through. With dealers long gamma, extensions away from the opening price tend to fade; with dealers short gamma, moves that began overnight tend to continue. A simple check is whether the index holds the range of the first thirty minutes. In a long-gamma session the mechanics favor the range holding, and a break often retraces toward the heaviest nearby strike; in a short-gamma session a break is more likely to extend.
3. The midday session
From late morning into early afternoon, charm, the change in delta as time passes, makes up a larger share of dealer activity. Under the usual index positioning, dealers are short protective puts and hold a short hedge against them, so as those puts decay they buy part of the hedge back. On a quiet long-gamma day this can show up as a slow drift higher that long-gamma hedging caps near the call wall. In a short-gamma session charm is small next to the hedging of price moves, so midday is often active rather than quiet.
News still moves the index at midday, and charm is one input among several. The link can be checked on a given day by comparing the midday drift with the direction and size of the charm flow implied by the open-interest map. When they disagree, other flows are in charge.
4. The afternoon and the close
Same-day options dominate the last part of the session. Their gamma gathers into a narrower band around spot as expiry approaches, and the concentration keeps rising until the close, so it peaks in the final hour. With dealers long gamma at a large same-day strike, pinning is strongest here and ranges can become very narrow. With dealers short gamma, hedging adds to any move and combines with the rebalancing of leveraged ETFs, which buy after up days and sell after down days near the close, so late moves can be large.
At the bell, the PM-settled options expiring that day leave the book with their gamma. On a monthly expiration, the standard AM-settled SPX options have already settled on the morning's opening prices. The next session starts from a different map, so the levels are worth reading fresh each morning.
5. Regime persistence from day to day
Gamma regimes tend to last because the positions behind them do. A long-gamma regime usually ends when a shock carries the index through the flip faster than positions adjust. A short-gamma regime usually ends when the index recovers above the flip, or when the puts behind it expire or are sold and new positions rebuild the long-gamma side.
Transitions are most plausible around events that change positions or inputs in bulk: monthly and quarterly expirations, which remove large amounts of open interest at once, and scheduled releases such as Federal Reserve decisions and inflation reports, which move both the index and implied volatility. Knowing when the next one falls is as useful as knowing the levels.
6. Reading the combinations
No single level is a signal on its own. These combinations describe what the hedging mechanics favor; news, or flows dealers do not control, can override any of them.
- Positive net gamma, spot below the call wall. Hedging leans against moves both ways, and under the usual positioning charm adds modest support. The mechanics favor a contained session that drifts toward the call wall rather than through it. If customers bought the same-day calls near spot, dealers are short them, and the approach to those strikes can be faster than the regime suggests.
- Negative net gamma, rising implied volatility, spot near the put wall. Gamma and vanna hedging both sell into weakness. The mechanics favor extension over reversal, and a bounce has less hedging support than it would above the flip.
- Spot holding one strike for an extended period in a long-gamma session. This is what a pin looks like. Hedging at the strike absorbs moves away from it, so breaking the pin usually takes a new catalyst.
- The session before a Federal Reserve decision. The regime suggests whether the reaction is more likely to fade (long gamma) or extend (short gamma). The decision also moves implied volatility, so vanna can redraw the map within minutes.
The DataFirst terminal shows these levels, so each condition can be checked against the current map. 0DTE gamma covers the same-day expiry, and SPX gamma exposure explained how the index map is built.
For educational and informational purposes only. Not financial advice. Options trading involves substantial risk of loss.