Understand market session phases — Stock-Market notes

WHY do session phases exist at all?

WHAT are the phases?

Figure — Understand market session phases

HOW to reason about each phase (derived, not memorised)

We can derive the expected behaviour from two simple quantities.

Formula Volatility as a function of information flow

Let $I(t)$ = rate of new information arriving, and $P(t)$ = number of active participants at time $t$ .

Step 1 — Why price moves: Price moves when orders are imbalanced. An imbalance requires (a) a reason (information $I$ ) and (b) someone to act on it (participation $P$ ).

Step 2 — Combine: The instantaneous volatility (size of price wiggles) scales with the product: $\sigma(t) \;\propto\; I(t)\cdot P(t)$ Why the product and not the sum? If either factor is zero, there is no volatility: news with nobody trading (holiday) → nothing moves; huge crowds but zero news (dead lunch) → nothing moves. A product is zero when either term is zero — a sum is not. Hence multiplication captures the "need both" logic.

Step 3 — Read off the phases:

Open: $I$ huge (overnight backlog dumps at once), $P$ high → $\sigma$ max.
Midday: $I$ low, $P$ low → $\sigma$ min.
Close: $I$ moderate but $P$ spikes (rebalancing/squaring) → $\sigma$ high again.

This reproduces the classic U-shaped (smile) intraday volume/volatility curve — high at both ends, low in the middle.

Worked examples

Common mistakes (steel-manned)

Active recall

Recall Explain like I'm 12 (Feynman)

Imagine a school day. Right when the bell rings in the morning, everyone is talking at once about what happened last night — super noisy and crazy. That's the market open. During lunch, half the kids are napping — quiet and boring; that's midday. Then the last-bell rush before going home, everyone scrambling to grab their stuff — loud again; that's the close. The market has a morning rush, a sleepy lunch, and a going-home rush. Prices jump around most during the two rushes.

Flashcards

Why is volatility highest at the market open?

Overnight information (

I

) piles up and is released all at once while many participants (

P

) act — the product

I\cdot P

peaks.

Why is intraday volume/volatility U-shaped?

High information + participation at open and high participation (rebalancing/squaring) at close, with low both at midday. Since

\sigma \propto I\cdot P

, both ends are high, middle is low.

What is the difference between pre-open indicative price and opening price?

Indicative price is provisional and updates as auction orders arrive; the opening price is the final auction equilibrium that begins continuous trading.

When should you route a large order to minimise slippage?

During high-participation phases (open/close) when the order book is deep.

Why does midday suit range/mean-reversion traders?

Low information and participation → low volatility → price oscillates in a band rather than trending.

Why do gaps appear at the open?

Overnight information is priced by the opening auction into a single print; with no continuous trading overnight, the jump shows as a gap.

Why model volatility as a product

I\cdot P

instead of a sum?

A product is zero when either factor is zero (news with no traders, or traders with no news), matching the real "need both" behaviour.

Connections

Liquidity and Bid-Ask Spread — why thin midday books cost you.
Slippage and Order Execution — choosing phases to fill big orders.
Opening Auction Mechanism — how the opening price is discovered.
Intraday Volume Profile — the empirical U-shape.
Volatility — the $\sigma \propto I\cdot P$ intuition generalised.
Gap Trading — trading the overnight-information jump.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, ek trading din ko ek uniform stream mat samjho — usme ek rhythm hota hai, bilkul school ke din jaisa. Subah bell bajte hi (market open) sab log raat bhar ki news, earnings, global markets ka reaction ek saath price mein daal dete hain — isliye volatility aur volume maximum hota hai. Dopahar ko (midday) sab thoda relax, news digest ho chuki hoti hai, bade institutions ruk jaate hain — isliye volume lowest, price bas ek range mein ghoomta hai. Phir closing ke time (last 30–60 min) funds rebalance karte hain aur log positions square-off karte hain — volume aur volatility phir se badh jaati hai.

Iska core logic simple hai: price tabhi hilti hai jab information (I) ho aur participants (P) us par act karein. Isliye volatility roughly $\sigma \propto I \times P$ . Product isliye, sum nahi, kyunki agar dono mein se ek bhi zero ho (news nahi ya trader nahi) to kuch nahi hilta. Isi wajah se din ka graph U-shape banata hai — dono kinaare high, beech mein low.

Practical baat: agar bada order daalna hai to open ya close choose karo, jahan liquidity zyada hai, taaki slippage kam ho. Breakout/trend trader ko open ka high movement pasand aata hai; range/mean-reversion wale ko midday ka calm behaviour. Aur haan — ek common galti: "midday calm hai matlab safe hai" — nahi bhai, calm ke saath wide spreads aate hain, to entry-exit cost zyada lag jaata hai. Phase ke hisaab se socho, ghadi dekhke andha trade mat karo.

Test yourself — When to Trade — Timing & Sessions

Connections