Why Do Option Returns Change Sign from Day to Night

Returns for S&P 500 index options are negative and large: -0.7% per day. Strikingly, when we decompose these delta-hedged option returns into intraday (open-to-close) and overnight (close-to-open) components, we find that average overnight returns are -1% while intraday returns are actually positive, 0.3% per day. A similar return pattern holds for all maturity and moneynesscategories, equity and ETFoptions, and VIX futures. Rational theories struggle to explain positive intraday returns. We show that returns change sign and become positive because option prices fail to account for the well-known fact that stock volatility is substantially higher intraday than overnight. Thus, option market-makers, some of the most sophisticated investors, appear to completely ignore one of the strongest volatility seasonalities, which can be easily added to option pricing models. Finally, other option investors also appear unaware of this anomaly, which may explain its persistence.