Charm — the decay of delta with time — accrues continuously, including over weekends and overnight. So while the sharpest charm flow hits into the Friday close, a subtler version operates across the whole run-up to a major expiration.
Why it drifts overnight
In the days before a big monthly or quarterly expiration, the large options book is steadily losing delta from charm. Dealers rebalance those hedges as time passes, and because meaningful time elapses overnight, some of that rebalancing shows up as a persistent overnight drift and a firmer open in the direction charm favors. It's a gentle, mechanical tilt rather than a violent move.
Charm doesn't sleep. The quiet pre-OPEX overnight drift is dealers rebalancing delta that time decayed while the market was closed.
The seasonal footprint
This is part of why the stretch into a monthly OPEX often has a characteristic feel — a supportive, low-drama drift — and why the character can change sharply after expiration, once that charm flow is spent and the positioning resets (the “OPEX unclench”). The tailwind was a hedging by-product, not conviction.
How to use it
Treat the pre-expiration drift as background context, not a trade trigger: it tilts the odds slightly and helps explain an otherwise-newsless grind, but it's small next to the intraday gamma regime. Its real value is expectational — knowing that the supportive pre-OPEX character can evaporate the moment expiration clears the book, so you don't assume the drift continues into the following week.