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.