Box fans are safe enough to run all night.

Box fans are generally safe to run unattended all night, but you should use common sense. Most modern box fans have thermal overload protection that shuts them off if the motor gets too hot. So the risk of a fire is pretty low—certainly lower than leaving a space heater on.

The real concern is dust buildup and airflow. If the fan is sitting on a dirty floor or has a thick layer of fuzz on the grille, the motor has to work harder and could overheat. Same deal if you block the back of the fan with a curtain or blanket. That’s more likely to kill the fan than start a fire, but still not great.

For an older fan with no safety certification (pre-2000s, no UL/ETL mark), I’d be more cautious. Those can fail in less forgiving ways. But a standard box fan from Home Depot that’s clean and in a clear spot? Run it all night and don’t lose sleep.

Just don’t let it suffocate under a blanket.