Learn about overnight and gap risk — Stock-Market notes

WHAT is a gap?

WHY does a gap happen? Price is set by the balance of buyers and sellers. When the market is closed, that balance is frozen, but new information keeps arriving. All the pent-up buy/sell decisions get released at the single reopening auction. If overwhelming demand arrived, the first executable price is far above the old close — so price teleports there instead of sliding continuously.

Figure — Learn about overnight and gap risk

HOW big is the risk? (Derive it from scratch)

We want the loss on a gap. Start from the definition of profit/loss on a long position of $N$ shares bought at price $P_{\text{entry}}$ .

Step 1 — Basic P&L. $\text{P\&L} = N \,(P_{\text{sell}} - P_{\text{entry}})$ Why this step? P&L is just shares times the price change; nothing else is going on yet.

Step 2 — Normal (intraday) stop works. If price falls smoothly, you sell exactly at your stop $S$ . Loss is bounded: $\text{Loss}_{\text{intraday}} = N\,(P_{\text{entry}} - S)$ Why this step? A stop-loss is a resting order; if price passes through $S$ during live trading, it triggers at (about) $S$ .

Step 3 — The gap breaks the stop. Overnight, price never trades at $S$ . It reopens at $P_{\text{open}} < S$ . Your stop is a market order once triggered, so it fills at the first available price, $P_{\text{open}}$ , NOT at $S$ : $\text{Loss}_{\text{gap}} = N\,(P_{\text{entry}} - P_{\text{open}})$

Step 4 — Define slippage from the gap. $\underbrace{\text{Loss}_{\text{gap}} - \text{Loss}_{\text{intraday}}}_{\text{extra loss}} = N\,(S - P_{\text{open}})$

Step 5 — Gap size as a percentage. $g = \frac{P_{\text{open}} - P_{\text{close}}}{P_{\text{close}}}\times 100\%$ Why this step? Traders compare gaps across stocks of different prices, so we normalize by the previous close.

Worked Examples

Forecast-then-Verify

Recall Before reading on, predict: If you double position size

$N$ but keep the same stop $S$ , does gap slippage $\Delta L$ double, quadruple, or stay the same? Answer: It doubles. $\Delta L = N(S-P_{\text{open}})$ is linear in $N$ . Doubling shares doubles every dollar of unexpected loss. This is why large overnight positions are dangerous — the gap term scales with size while your control does not.

Common Mistakes (Steel-manned)

Managing the Risk (the 80/20)

Flashcards

What is a gap in price?

A discontinuity where the next session's open differs from the prior close, with no trades in between.

Define overnight risk.

The risk that an open position changes value while the market is closed and cannot be traded/adjusted.

Why can a gap "skip" your stop-loss?

A stop triggers only when price trades through it; overnight price jumps over it and fills at the open instead.

Formula for actual loss on a gap-down long?

N(P_{entry}-P_{open})

Formula for gap slippage (extra loss beyond stop)?

\Delta L = N(S - P_{open})

If you double position size, how does gap slippage change?

It doubles —

\Delta L

is linear in

N

Percentage gap formula?

g=(P_{open}-P_{close})/P_{close}\times100\%

One reason big stocks can gap more than penny stocks?

Scheduled catalysts like earnings that everyone is holding into.

The single surest way to eliminate overnight gap risk?

Hold no positions overnight (flatten before the close).

Recall Feynman: explain to a 12-year-old

Imagine you're playing a video game where you can only move your player during recess. At night the game keeps changing the map — but you're asleep and can't touch anything. In the morning your player might be standing somewhere totally new: maybe in a treasure room (yay), maybe in lava (ouch). You even left a "run away if lava gets close" rule — but the game teleported you straight into the lava, so your rule never got a chance to fire. That teleport is a gap, and the danger of sleeping while the map changes is overnight risk.

Connections

Stop-Loss Orders — why they fail across gaps.
Position Sizing — the $N$ term that scales your gap loss.
Earnings Season — the #1 scheduled gap catalyst.
Day Trading vs Swing Trading — flat-by-close removes overnight risk.
Volatility & ATR — expected size of the overnight move.
Options as Hedges — capping loss through gaps.
Pre-Market & After-Hours Sessions — where the news gets priced before the open.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, market sirf kuch ghante ke liye khula rehta hai, lekin duniya 24 ghante chalti rehti hai. Raat ko earnings aa jaayein, koi news aa jaaye, ya foreign market gir jaaye — to price bina rukе agle din seedha jump kar deta hai. Is jump ko gap kehte hain, aur band market ke doraan position hold karne ke risk ko overnight risk kehte hain.

Sabse important baat: log sochte hain "mere paas stop-loss hai, main safe hoon." Galat! Stop-loss tabhi chalta hai jab price us level ko cross karte hue trade kare. Gap mein to price us level ko skip kar deta hai — seedha neeche open hota hai. Isliye tumhara stop $48 pe tha, par actually fill hua$ 44 (open price) pe. Extra loss ka formula simple hai: $\Delta L = N\times(S - P_{open})$ . Yaani jitna gap bada, utna extra nuksan — aur jitni badi position ( $N$ ), utna zyada.

Practical baat 80/20 wali: overnight gap ko delete nahi kar sakte, sirf apna exposure kam kar sakte ho. Position size chhoti rakho, ya market band hone se pehle sab bech do (pure day-trading), ya options se hedge karo, ya earnings jaise scheduled events pe single stock hold mat karo. Yaad rakho — "Gaps Jump, Stops Trip" aur "Closed matlab Calm nahi hota." Screen pe raat ko kuch nahi hilta, iska matlab risk nahi hai — aisa nahi; risk chhupa hua hota hai, subah dikhta hai.

Test yourself — When to Trade — Timing & Sessions

Connections