Intraday Volatility Seasonality
The recurring, time-of-day pattern in how volatile a market is — high near the open and close, quieter through the middle of the trading day — and why strategies need to account for it.
Prerequisites: Standard Deviation
Volatility within a single trading day is not flat — it traces a well-known "U" or "smile" shape. Trading is typically most volatile in the first thirty to sixty minutes after the open, as overnight news and order imbalances get absorbed, quiets down through the middle of the day when fewer participants are actively repricing, and then picks up again into the close as funds rebalance and execute against benchmarks. This pattern is remarkably stable day to day, which is why it's called seasonality rather than noise — it repeats predictably enough to plan around.
Ignoring this shape causes two common mistakes: a risk model that assumes constant intraday volatility will misprice risk at the open and close, and a strategy that measures its edge using an average volatility number risks systematically over- or under-trading during the specific hours it's actually active. A market-making desk, for instance, typically widens quoted spreads in the first and last thirty minutes precisely because realized volatility is two to three times higher there than at midday, and a spread sized for the calm middle of the day would lose money during the open.
Intraday volatility follows a predictable U-shaped pattern — highest at the open and close, lowest at midday — and strategies, risk models, and execution schedules that assume flat volatility throughout the day will misjudge risk and cost during the periods when it matters most.
Further reading
- Andersen & Bollerslev, 'Intraday Periodicity and Volatility Persistence in Financial Markets', J. Empirical Finance 1997