Other organic debris
Over time, this material can form a slimy layer known as a biofilm.
That biofilm can provide food and shelter for developing larvae.
So when you repeatedly see drain flies around one particular sink or shower, it may be worth investigating that drain.
Does Seeing Drain Flies Mean Your Bathroom Is Dirty?
Not necessarily.
This is an important misconception.
Even a bathroom that is cleaned regularly can develop organic buildup inside plumbing.
A drain may look perfectly clean from above while the inside of the pipe contains material that you cannot see.
That's why simply wiping the bathroom counter or spraying an insecticide on the adult flies may not solve the problem.
If the breeding site remains, new adults can continue to appear.
The key is addressing the source rather than only killing the visible insects.
Why Do They Like Bathrooms So Much?
Bathrooms provide several things drain flies need:
Moisture
Bathrooms naturally contain water from showers, sinks, toilets, and condensation.
Organic Material
Hair, soap residue, skin particles, and other debris can accumulate inside drains.
Protected Breeding Areas
The inside of a drain can provide a relatively sheltered environment where larvae can develop.
Stable Conditions
Indoor plumbing environments can remain relatively stable compared with outdoor environments.
That combination makes bathrooms particularly attractive to these insects.