It is OK to leave a fan on overnight and sleep with it on.
I leave my fan on overnight and have been doing so for many years to help keep me cool and help me sleep and it's never been an issue.
Sleeping with a fan on all night will cause no harm although if you have an illness such as the flu, cold etc then the fan may make the illness worse but you can't get sick simply from sleeping with a fan on.
Around 20,000 house fires in an 8 year time period have been caused by fans.
Most of those house fires are a result of overheated fans, dirty fans that cause them to overheat, bad cords, lose outlet.
If you're gonna be gone longer than a day it's best to leave the fan off but if you're gone for a short while but are gonna return it's okay to leave the fan on such as for a pet.
If nobody else is in the house or any pets then it's best to unplug the fan as they only cool a person or pet and not the home.
You can leave a fan running 24 hours a day, 7 days a week as long as the fan is in good condition and you keep the fan clean and free of dust, debris etc.
Wash your fan with some water and blow any dust off to keep it in good condition and to prevent the fan from spreading dust around and check the cords etc to ensure they are in good condition.
Also make sure the fans cord fits snugly into the electrical outlet to prevent overheating.
If the fan is in good condition, the cord is in good condition and the outlet is in good condition then there should be no issues running the fan 24 hours a day.
Leaving a fan on all night can cause a fire if the fan is not in good shape or you have a frayed cord etc.
However simply leaving a fan on all night or 24 hours a day is not gonna cause a fire as long as the cords are in good shape, the fan is in good shape etc.
The fan when it gets too hot will shut down to prevent a fire.
A room fan or any fan can catch fire.
However it's rare that a fan catches fire as long as you don't have the cords overloaded, have a good outlet, have a good cord and your electric room fan is in good condition.
As long as you have a good fan, have a good non frayed cord and a good outlet there should be no worry about a fan catching fire.
Fans are meant to run for several hours and even overnight without any issues.
I have my electric fan running all night without any issues.