Why Buying in Daytime Sessions Hurts: The Cost of Gaps
Buying early in daytime ATS looks convenient, but overnight gap-downs straight through stops erode strategy edge.
For Korean traders, daytime sessions (Blue Ocean ATS, 09:00–17:00 KST) allow buying US equities during the local business day. It seems convenient to get fills early without staying up late.
However, backtests show that even with zero spread, daytime buying underperforms regular session (RTH) open fills. Why?
RTH open vs daytime ATS fills (S&P 500)
| Entry Method | Trades | Win% | E[R] | CAGR | MDD | MAR |
|---|---|---|---|---|---|---|
| RTH Open (Baseline) | 950 | 42.2% | 0.148 | 8.54% | −15.25% | 0.56 |
| Daytime (0bp spread) | 963 | 42.3% | 0.153 | 8.40% | −19.22% | 0.44 |
| Daytime (15bp LP spread) | 982 | 40.6% | 0.095 | 5.00% | −22.52% | 0.22 |
| Daytime (40bp spread) | 1011 | 38.5% | 0.032 | 2.66% | −26.16% | 0.10 |
The cause: Gap-through stops
Naive backtests assume an exit occurs at the exact stop price ($95). In reality, if bad news hits overnight and the market opens at $90, the fill happens at $90, incurring a 2R loss instead of 1R.
Buying in the daytime session carries an extra overnight gap. Accounting for realistic gap execution:
| Entry Method | Total Stops | Gapped Through | Gap Rate | Extra Loss/Trade |
|---|---|---|---|---|
| RTH Open Buy | 506 | 84 | 16.6% | 0.28R |
| Daytime Buy | 509 | 96 | 18.9% | 0.40R |
While the stop count is nearly identical (506 vs 509), daytime positions suffer deeper slippage on stops, adding an extra 0.40R loss per stopped trade. Maximum drawdown widens from −15.2% to −19.2%.
Policy conclusion
Minervini's SEPA depends on keeping losses small. Gap slippage expands 1R stops into 1.4R losses.
- Scanner New Buys: Restricted to RTH Open only.
- Stop Exits: Real-time execution across all sessions when spreads are tight (≤15bp); queue for RTH when books are thin.
- ETF DCA Sleeves: Allowed across daytime and pre-market sessions because scheduled accumulation carries no stop to gap through.