Quick Answer:
Most often this comes from the sink trap losing its water seal overnight. Before panicking, pour a cup of water into the drain at night and check in the morning. If the smell is much weaker, the trap is drying or being siphoned. Also listen for gurgling when other fixtures run — that can mean the trap is being pulled dry.
Why This Happens
Water in the trap (the U-shaped bend under the sink) blocks sewer gases. If that water evaporates or is siphoned away, gases come up the drain and you smell them first thing in the morning because the sink has not been used for hours. Changes in plumbing, blocked vents, or long periods of low humidity can make this more likely. If the odor began after being away, compare the situation to Food odor after long vacation. If it started after recent work on pipes, consider Odor after plumbing repair.
Step-by-Step What to Do
1. Do the night water test
- Before bed, pour a cup of clean water straight down the drain.
- In the morning, smell the sink area. If the smell is much reduced or gone, the trap lost its water seal overnight.
2. Watch for gurgling
- While someone else runs another fixture (toilet, washing machine or shower), stand near the sink and listen.
- Gurgling or bubbling from the sink drain while other fixtures run is a sign the trap is being pulled dry or the venting is not working properly.
3. Check for visible issues
- Look for slow drainage, leaks at the trap, or loose connections — these can let the trap lose water or allow sewer gas to escape.
- Clean the sink and the visible drain parts; food debris in the drain or disposal can add to morning smells.
4. Try simple fixes
- If evaporation seems likely, a routine of running water before bed or leaving the tap dripping slightly overnight can help temporarily.
- If the smell persists despite keeping water in the trap, the problem is likely siphoning or a vent issue and needs further diagnosis.
What Not to Do
- Don’t ignore a trap that’s repeatedly going dry — masking the smell doesn’t fix siphoning.
- If gurgling happens when other fixtures run, venting issues are best handled by a plumber.
- Don’t pour strong chemicals down the drain to “get rid of the smell” without knowing the cause; they can damage pipes and create hazardous fumes.
When to Call a Professional
- If the night-water test doesn’t reduce the smell.
- If you hear gurgling or bubbling when other fixtures run, or multiple drains are affected.
- If you find leaks, loose trap parts, or can’t trace the odor to the sink area.
- If the smell is strong and persistent — sewer gas can be a health concern and needs proper diagnosis.
Safety Notes
- Sewer gas can contain harmful compounds. If the smell is strong or you feel lightheaded, leave the area and get fresh air.
- Use gloves when working under the sink and avoid mixing cleaning chemicals. If you’re unsure, stop and call a plumber.
- Don’t attempt major venting or sewer-line work yourself; these are best left to licensed professionals.
Common Homeowner Questions
- Q: Is it normal for a sink to smell only in the morning? A: Yes, it often means the trap has lost its water seal overnight from evaporation or siphoning.
- Q: Can I just pour water in every night? A: You can as a temporary measure, but repeated drying suggests a plumbing issue that needs attention.
- Q: Could the smell be from food or the disposal? A: Yes; food buildup can add to smells. Clean the drain and run the disposal, but if the smell returns after the night test, suspect the trap or venting.
For more related articles, see the Food Waste Odors & Bio-Buildup hub.