Vega measures how much an option's price moves for a 1-point change in implied volatility. It's the greek that makes longer-dated options swing when the vol environment shifts. On a 0DTE contract, it's nearly irrelevant — and that's a feature worth understanding.

Vega scales with time

Volatility is about how far price can travel over the remaining life of the option. With weeks left, a change in expected volatility has lots of time to matter, so vega is large. With hours left, there's almost no runway for vol to play out, so vega collapses toward zero. A same-day at-the-money SPY option has a fraction of the vega of a monthly.

What this means in practice

On longer-dated options, an IV crush after a catalyst can gut your position even if you're right on direction — that's vega at work. On a 0DTE option, a drop in IV barely registers through vega, because there's almost no vega to hit. That removes one of the classic ways to lose while being right. But it doesn't make 0DTE safer overall — it just changes which greek is dangerous.

On 0DTE, vega goes quiet and gamma and theta take the wheel. You're not fighting the vol environment — you're fighting the clock and the whip.

The greeks that take over

With vega muted, a same-day option's price is dominated by gamma (the explosive at-the-money whip) and theta (the relentless time bleed). That's the 0DTE trade in one line: little vol sensitivity, enormous move-sensitivity, and a clock that never stops. Trade it knowing which forces are actually on the field.