There is a sewage smell in the bathroom sink after it sits especially in the morning:cool: :confused:

Check the P trap beneath the sink. It should be full of water and that prevents sewer gas from getting inside then home. This may be mold inside the overflow cavity but mold does smell like poo. Are you sure it's a sewer gas smell?

Are you flushing the toilet after using the sink when you go to bed?
When you do flush the toilet, do you hear a gurgling sound coming from the sink?

You could have a clogged stink pipe. When sewage goes down a drain, air needs to be drawn in so the waste can make it down to the sewer or septic field. The stink pipe provides that air supply. If the stink pipe is clogged (with a bird's nest, leaves, fruit hidden by squirrels, etc.) air will be drawn in through another source: like your bathroom sink. Gasses can then come back up making your bathroom smell like, uh, poop.

Write back and let us know if that gurgling sound is happening.

Most bad smells from a lavatory come from hair built up on the plunger rod and decaying down there. Every time you open the stopper to drain the lavatory you get a nose full of stinky.
The first thing I would look at is the stopper. Pull it and you will find a rod about 6" down in the drain. Shine a light down there and if you see a buildup of hair fish it out. If the stopper's tied down let me tell you how to free it up. Push down on the plumger so that the stopper pops up. Underneath the lavatory ahead of the clip you will see a knurled nut,(Leave the clip on)Now back the nut out until it's free but don't pull it all the way out. Now while you are pulling up on the stopper GENTLY pull the rod back until the stopper clears, stop! Now screw the nut back in place and just snug it up with a pair of pliers. When you clear the drain just drop the stopper back in. Some plumbers lock the stopper in when they install the fixture. I never have for it makes it impossible for the home owner to clean the drain. After you clean the hair pour a 1/2 gallon of bleach down the drain and let it set overnight without using it. Next morning flush it down with a pan full of boiling water. That should take care of your smelly problem. Hope this helps and thank you for rating my reply. Tom