NIFTY Expiry Day, Second by Second: Gaps, Strangle Survival and the Real Math of the Short Straddle

What this study is, and why we rebuilt it

Most expiry-day option studies stop at where the index settled. That hides the part a premium seller actually lives in: the path. Using second-by-second NIFTY spot from April 2018 to June 2026 — 44 million ticks — together with a minute-level ATM option chain, we rebuilt the standard expiry-day analysis and added the questions the settlement-only view cannot answer.

The sample is 148 NIFTY weekly expiries (days-to-expiry zero) from July 2023 to May 2026 — every expiry that carries both clean 1-second spot and ATM premium data. Three findings reframe the usual story: a strangle is breached intraday far more often than the settlement number implies, the overnight gap is a tradeable signal, and the famous 86% straddle decay is mostly given back to directional moves once you hold a real fixed strike to expiry.

Everything below is spot- and premium-based, gross of costs. It is research for educational use, not investment advice.

Expiry-day spot moves

The typical NIFTY expiry closes slightly red. The median move is -23 points and 85 of 148 expiries finish down, but the mean is only -12 — a handful of sharp up days (63 up days averaging +123) pull the average back toward zero. It is a right-shouldered distribution with a red median.

Risk is tail-driven. The worst single expiry (-575 points) is nearly double the 95th-percentile move (±306). For a short-premium book, the rare day, not the quiet majority, decides survival.

Short-strangle survival: settlement versus intraday touch

The original question — how often does a strangle expire worthless — only looks at the close. With 1-second data we can also ask whether the strike was ever touched intraday. That is the difference between a strangle that quietly expires worthless and one that breached your strike, hit a stop, or spiked your margin before settling back.

At settlement the long-run edge is intact: a ±200 strangle finishes worthless 84% of the time, ±300 94%, ±400 99%. But on a touch basis those numbers fall hard — ±200 drops to 66% and ±150 to 50%. Between a fifth and a third of strangles that 'expired worthless' were breached at some point during the day.

The put side breaks more often at every width — the down-drift showing through — so a put-skewed strangle (put strike one step further out) equalises the side risk. The practical rule: if you run stops or face intraday mark-to-market margin, size to the touch column, not the settlement column.

Intraday ATM straddle decay

The rolling ATM straddle falls from about 144 points at the 09:15 open to 21 by the close — an 86% gross bleed, and the reason expiry day attracts premium sellers.

The curve is front- and back-loaded. Roughly 18% of the open value evaporates in the first 15 minutes as the opening auction settles, the middle of the day glides (42% gone by noon), then decay accelerates into the close as gamma and theta spike. The 11:00–13:30 window is the slowest per minute.

One caveat sets up the most important section: this measures the rolling ATM straddle, which re-centres as spot moves. A real seller holds a fixed strike. How much of this decay reaches the P&L is a very different number.

Opening gaps, part 1: gaps mean-revert

Expiry-day opening gaps average +20 points (median +17), with 89 gap-ups against 59 gap-downs and a wide range (-460 to +1210).

Gap and expiry move are negatively correlated (r = -0.24). Only 43% of expiries continue in the direction of the gap — the majority reverse. Gap-ups fade (mean move -40) and large gap-downs bounce (median +74). On expiry day, the session leans toward filling the overnight move rather than extending it.

Opening gaps, part 2: they usually fill, and fast

68% of expiry-day gaps are filled — price returns to the prior close at some point intraday (70% of gap-ups, 66% of gap-downs). The median time-to-fill is about 12 minutes, and 80% of fills happen before roughly 10:30.

But fill probability collapses with gap size. Small gaps under 0.1% fill 89% of the time; gaps larger than 0.5% fill only 25%. The big gaps are the trend days — exactly the days a fade or a naked short strangle is most dangerous.

Opening gaps, part 3: which leg breaks, and whose straddle bleeds

The gap also predicts the side at risk. On large gap-up expiries the put strike (±150) is touched 39% of the time versus only 12% for the call — the index gaps up, then fades down through the put. Large gap-downs reverse it: the call is touched 35% versus 24% for the put. The actionable read is to skew the strangle against the gap — wider on the side the day is likely to fade into.

Decay is not gap-neutral either. Gap-down expiries bleed the fastest: by the close their straddle is worth about 11% of the open versus roughly 17% on gap-ups, and they are already cheaper by noon. A gap-down that bounces back up is the premium seller's best decay environment of all.

The real economics of the short straddle

The 86% decay figure is gross. To get the number that actually matters, we sell a fixed ATM straddle at the 09:15 open and hold it to expiry, booking the true payoff: premium collected minus the absolute distance from the strike at the close.

The result is a textbook win-small, lose-big profile. The straddle is profitable on 69% of expiries but only banks about +80 points on a winning day, while the 46 losing expiries cost -91 on average and -290 at worst. The net mean is +27 points — roughly 19% of the premium collected, not the 86% the gross-decay curve implies. The other two-thirds is given back to directional moves.

That single number reframes the strategy. The edge is real, but it is thin and tail-exposed, which means tail management — stops, day-level loss caps, event-day avoidance, position sizing — moves the needle far more than chasing a marginally higher worthless-rate. And because the open captures the most premium against the same terminal payoff, a held-to-expiry straddle is best entered early; capturing the steep late-day decay instead requires a short-hold afternoon leg that is bought back before the close, not carried into settlement.

What it means for an expiry-day seller

Budget for the give-back. Gross premium decay near 86% overstates what a held-to-expiry straddle keeps (about 19%). Plan position size and return expectations around the captured edge, not the decay headline.

Size to the touch column, not settlement. Intraday breach rates are materially higher than settlement worthless-rates; stops and margin live in the intraday world.

Trade the gap. Gaps mean-revert and usually fill, so fade modest gaps and skew the strangle against the gap. Treat gaps larger than 0.5% as trend-day warnings — only a quarter of them fill.

Lean against the down-drift. A put-skewed strangle equalises the side-breach asymmetry that shows up on flat and gap-up days alike.

Data, methodology and limitations

Spot is NIFTY index 1-second OHLC, April 2018 to June 2026. Expiry-day open is the 09:15:00 tick, close is the last tick at or before 15:30, and the overnight gap is the open minus the prior session's final tick. A strike counts as touched if the intraday high or low reaches it. The ATM straddle premium comes from a minute-level option chain; the 148 expiries used are those with both clean spot and premium data.

Limitations: the spot sections carry no premiums or costs, and percent-worthless is a survival frequency, not a P&L. The strangle results are spot-derived because OTM premiums were unavailable, so only the ATM straddle carries true premium P&L, which is itself gross of slippage and fees. ATM is approximated by the 09:15 open rounded to the strike grid. The sample spans a broadly trending, low-volatility regime, so the tail figures are the least stable numbers here, and the gap mean-reversion fit is influenced by a few very large gaps — the bucketed table is the more robust read.