Theta — the daily decay of an option's time value — is not a straight line. On expiration day, for an at-the-money option, it curves sharply downward into the close. Understanding why changes how you trade the afternoon.

Time value has less and less time

An option's extrinsic (time) value must reach zero at expiration. As the hours tick down, that remaining value has to disappear over a shorter and shorter window — so the rate of decay increases. Early in the day the melt is gradual; in the last couple of hours it's a cliff (theta decay basics).

Why at-the-money is worst

The acceleration is most brutal for at-the-money options, which are almost pure time value. A slightly in- or out-of-the-money option has intrinsic value or is already nearly worthless; the ATM contract has the most extrinsic value to shed in the least time. That's exactly the strike a 0DTE scalper tends to trade.

In the last hour, an ATM 0DTE option is a melting ice cube. Being right on direction isn't enough — you have to be right *fast*.

What it means for your trades

Afternoon 0DTE trades need to work quickly — the decay clock is running faster than your intuition says. This is the entire logic behind a time-stop: cut a stalled position before theta does it for you (0DTE risk management). It's also why gamma and theta are locked in a tug-of-war at expiration (theta vs gamma). See the best time of day to trade 0DTE.