Most of the time, the culprit is a biofilm of bacteria living in your drain. These little guys love munching on soap scum, hair, and dead skin cells—delicious, right? As they feast, they produce hydrogen sulfide gas, which is literally the same stuff that makes farts and rotten eggs smell. So when you turn on the water, that gas gets pushed up into your face. (Not exactly the wake-up call you asked for.)
This is why the smell often hits hardest after you haven’t used the sink for a while—like overnight. The bacteria have had hours to party in your pipes, and the gas has built up. It’s not a sign your water is poison, but it is a sign your drain needs a little TLC.
Hold On—Could It Be the Water Itself?
Sometimes, the smell comes from your water heater, not the sink drain. If you only notice the rotten egg odor with hot water, you might have a magnesium or aluminum rod inside your heater reacting with sulfur in the water. Yes, water heaters have sacrificial rods—and yes, they can start smelling like a chemistry lab gone wrong. Flushing the tank or swapping the rod can fix it. (You didn’t think you’d escape the weekend without a plumbing project, did you?)
And get this: well water naturally contains hydrogen sulfide in some regions. So if you live in the countryside and your bathroom sink smells like a swamp monster’s armpit, it might be your well’s natural chemistry. In that case, a simple carbon filter or aeration system can make that water smell sweet again. Or at least neutral.
