0DTE, charm and vanna: why the last hours before expiry behave differently

August 14, 2026

Gamma is not the only Greek that generates hedging flow. Charm turns the passage of time into buying and selling, vanna turns changes in implied volatility into the same, and both peak exactly where 0DTE trading lives. A plain-language explanation.

Same-day expiry options now make up a large share of index option volume, and the sessions they dominate have a distinct texture: long quiet stretches, then sharp moves into the afternoon, then price settling onto a round number.

That texture is not mysterious. It comes from the fact that two other Greeks also generate hedging flow, and both of them reach their maximum in the final hours of an option's life.

If you have not read what dealer gamma is, start there. This post assumes the hedging mechanic.

Why 0DTE gamma is so extreme

Gamma measures how fast delta changes. For an option with weeks left, delta moves gradually: a $1 move in the stock nudges it.

For an option expiring in two hours, delta is close to a step function. Slightly in the money, the option will almost certainly be exercised, so delta approaches 1. Slightly out of the money, it will almost certainly expire worthless, so delta approaches 0. A small move in the underlying flips the option between those states.

That is enormous gamma, and it is concentrated in a narrow band around spot. Strikes 2% away have almost none, because nothing plausible in the remaining hours changes their outcome.

Two consequences follow directly:

Hedging flow near the money becomes very large per point of movement. The dealer's required position changes fast, so the hedge is adjusted fast.

Gamma at distant strikes evaporates as the day progresses, without a single contract trading. The whole gamma profile narrows toward spot over the session.

Charm: the passage of time as a flow

Charm is the rate at which delta changes with time. Formally the derivative of delta with respect to time to expiry.

The intuition is easier than the definition. Take an out-of-the-money call with a delta of 0.20. Hold the stock perfectly still and let three hours pass. That delta drifts toward zero, because there is now less time for the stock to reach the strike. Nothing happened in the market, and the option became less stock-like.

A dealer hedging that option was holding shares against a 0.20 delta. Now they need shares against a 0.12 delta. They sell the difference. Time alone produced a trade.

Across an entire chain, charm generates a persistent flow whose direction depends on the shape of the dealer's book. Into a Friday expiry or a monthly opex, the accumulated unwinding of hedges against options that are quietly dying is a real, mechanical source of order flow with no news attached to it.

This is the honest core of the "pin into expiry" idea. Options decaying toward zero delta cause hedges to be unwound, and if the surviving concentration is near a particular strike with dealers long gamma, the residual hedging oscillates around it.

Vanna: implied volatility as a flow

Vanna is the rate at which delta changes with implied volatility.

Consider an out-of-the-money put. When implied volatility is high, that put has meaningful delta: a big move is plausible, so it behaves somewhat like short stock. When implied volatility falls, the same put's delta shrinks toward zero, because the market now regards that outcome as unlikely.

Now apply the hedging logic. A dealer short that put is hedged with a short stock position. Implied volatility falls, the put's delta shrinks, the required short is smaller, so the dealer buys stock back.

That is the mechanic behind a familiar market pattern: a decline in implied volatility, with no change in the underlying, mechanically produces buying. It is one credible explanation for the grinding drift higher that often follows a volatility spike, once the spike subsides.

The reverse holds. Rising implied volatility increases put deltas and forces hedging sales, which is part of why volatility expansions can feel self-reinforcing.

Why these two matter most at expiry

Charm and vanna are both largest when options are near the money and near expiry, which is precisely the 0DTE region.

That produces the characteristic shape of an index session dominated by same-day expiry:

Morning: the widest gamma profile of the day, several strikes still live,

moves can travel before running into concentrated hedging.

Midday: distant strikes lose their gamma. The profile narrows. If dealers

are long gamma near spot, this is where absorption is strongest and ranges compress.

Afternoon: charm accelerates. Hedges against dying options unwind. The

surviving gamma is a narrow spike around one or two strikes.

Close: the residual concentration dominates, and price often settles near

it.

Note the conditional in the midday step. All of this depends on the sign. If dealers are short gamma near the money instead, the identical clock produces the opposite session: narrowing gamma amplifies rather than absorbs, and the afternoon gets more violent instead of quieter.

The 0DTE clock is not inherently a pinning mechanism. It is an intensifying mechanism, and what it intensifies is whichever regime is in force.

Four cautions

These are model quantities. Charm and vanna come out of an option pricing model with an implied volatility input. Change the surface and you change the numbers.

The sign assumption still applies. Every statement above rests on an assumption about which side dealers hold, which is not observable from open interest.

Magnitudes are routinely overstated. These flows are real. Whether they dominate a given session, versus ordinary supply and demand, is an empirical question and the answer varies enormously by day.

The mechanic is still not directional. Vanna's effect depends on the direction of the volatility change, and charm's depends on the shape of the book. Neither says which way price goes tomorrow.

The useful summary

Gamma tells you how strongly hedging reacts to price. Charm tells you that time alone generates flow, and vanna tells you that changes in implied volatility do the same. All three peak in the same place, which is why the final hours before expiry have their own character.

Treated as a description of session structure, this is among the better-founded frameworks available. Treated as a forecast, it is being asked for something it does not contain.