USB extension cords are fine overnight.

Yes, if you buy a half-decent one.

The risk isn’t really about overnight charging itself — it’s about quality. A UL-listed or similar certified extension cord with USB ports is safe to leave plugged in. The power conversion is built-in and should have overcurrent and overheating protection. The same safety rules apply as for any other charger: don’t bury it under blankets, don’t run it through a wall, and don’t use it if it feels hot.

The problem is the cheap knockoffs. No-name brands from dubious marketplaces sometimes skip safety certifications to save pennies. Those can have poor voltage regulation or missing fuses. That’s where the fire risk lives.

So buy from a brand you’ve heard of — Belkin, Anker, GE, Leviton — and you’re fine. The USB port is just a wall wart you don’t have to carry around.

Don’t cheap out on the thing that handles power while you sleep.