A purifier without a fan will run more quietly than a purifier with a fan. However, a unit without a fan won’t circulate air as effectively as a unit with a fan. As such, you’d be trading the quiet of a fan-less purifier for a less efficient machine. Instead, consider purchasing an air purifier with multiple fan levels. You can turn down the fan’s speed to reduce noise while sleeping. And you can set the fan at higher speed when more extensive air cleaning is needed.