Toilet runs only at night

Toilet running noise noticeable only at night.

Quick Answer:

If you hear the toilet running only at night, it’s commonly a slow leak inside the tank that lets water escape into the bowl and causes the fill valve to cycle. Do a dye test in the tank before bed; if color appears in the bowl without flushing, the flapper/seat is leaking and cycling the fill. That simple test will tell you if the tank seal is the likely cause.

Why This Happens

At night the house is quiet and small, intermittent sounds are easier to hear. A slow flapper or a bad flapper seat can let water pass from the tank into the bowl slowly; when the tank level drops enough, the fill valve opens and refills the tank, making the running sound. Temperature and slight pressure changes overnight can make a small leak more noticeable then.

If the running happens at other odd times or unpredictably, you may also want to read about Toilet runs randomly for troubleshooting that covers intermittent behavior.

Step-by-Step What to Do

1. Gather a few supplies

  • Food coloring or a toilet dye tablet
  • Flashlight
  • Towel or small bucket (for any water you move)
  • Optional: basic replacement flapper if you plan to swap it out

2. Do the dye test before bed

Flush to clear the bowl, then add several drops of food coloring into the toilet tank. Wait without flushing and go to bed. If color appears in the bowl in the morning without anyone using the toilet, the flapper/seat is leaking and cycling the fill.

3. Inspect the flapper and seat

  • Remove the tank lid carefully and look at the flapper when the tank is full. Check for cracks, warping, or mineral buildup on the flapper and the seat.
  • Press the flapper down manually; if the leak stops, the flapper or its chain is likely the issue.

4. Check the fill valve and overflow

  • Make sure the overflow tube is at the correct height and the water level is about 1 inch below the top of the tube.
  • If water is trickling into the overflow constantly, the fill valve may need adjustment or replacement.

5. Try a simple fix, then retest

  • Clean mineral deposits from the flapper and seat with a gentle scrub or replace the flapper if it looks damaged — this is an inexpensive, common fix.
  • After any adjustment or replacement, do the dye test again and monitor overnight to confirm the issue is resolved.

6. Track what you find

Note when the running started, whether it follows any plumbing work, and whether the dye test was positive. This information helps if you call a professional.

What Not to Do

  • Don’t assume night-only running is “normal settling”—it often means a slow flapper leak; if dye confirms, a plumber is appropriate.
  • Don’t use strong chemical additives in the tank to try to stop a leak; they can damage rubber parts and make repair harder.
  • Don’t overtighten or force plastic parts when you inspect them; that can break the tank or fittings. If you’re unsure, stop and call a pro.

When to Call a Professional

  • If the dye test is positive and replacing or cleaning the flapper doesn’t stop the cycling.
  • If the leak is intermittent and you cannot reproduce it reliably, or if it began after other plumbing work—mention details about the repair so the plumber can diagnose faster. If the behavior started immediately after service, also consider Running toilet after plumbing repair when explaining the issue.
  • If you notice a sudden large continuous flow, unusually high water bills, or multiple fixtures acting up at once.

Safety Notes

  • Turn off the toilet shutoff valve (behind the toilet) before doing any part replacement to avoid accidental flooding.
  • Lift the tank lid carefully and set it on a flat surface to prevent it from falling and cracking.
  • Use gloves when handling old rubber parts or mineral deposits. Avoid inhaling dust from scraping away buildup.
  • If you must turn off the house main water or work on supply lines and you’re not comfortable, call a plumber.

Common Homeowner Questions

  • Why is the running only noticeable at night? — Nighttime quiet makes small, intermittent runs more audible and small leaks may be triggered by cooler temperatures or pressure changes overnight.
  • Can I ignore a small nightly run? — No. A slow leak wastes water and can get worse; do the dye test and address the cause.
  • Is replacing the flapper a DIY job? — Yes, replacing a flapper is usually simple and inexpensive, but call a plumber if you’re unsure or if the leak persists after replacement.

For more related articles, see the nan hub.

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