“What causes a ringing sound in the ears?”

This is called tinnitus and can be caused by a variety of reasons. Most commonly, tinnitus is a symptom of hearing loss. For this reason, if she has not had a recent hearing exam by an audiologist, I suggest she have this done. Also, tinnitus is a side-effect of many medications and supplements.

The reason most people find it bothersome at bedtime is because that is when our homes are the most quiet, so the tinnitus is more noticeable.

For most people, tinnitus can be mildly bothersome, but not debilitating. If your mother experiences worsening tinnitus or it seems worse on one side, she should speak with her physician right away.

This is usually a sign of hearing loss. She should see an audiologist and take the necessary steps to treat her hearing loss (usually this means prescription hearing aids), and most likely she will get relief from the ringing you describe. Feel better!

High frequency hearing loss is the #1 cause. I recommend she have a hearing test and pursue hearing aids. 

Ringing in the ears is often a symptom of hearing loss. The brain creates phantom sensations when we acquire sensory impairments. For example, people who lose a limb feel like it's still present - sometimes it even hurts or itches - which has been labeled "phantom limb syndrome." This ringing usually worsens in quiet environments because there's no external stimulus to distract from the internal phantom sound. It becomes a real problem that needs to be addressed when the ringing starts to interfere with sleep or concentration, creates stress or anxiety, or impacts your normal daily life. Most people notice significant improvement during the day by using hearing aids to give the brain back some of the sound that it's been missing. At night, using a sound machine with steady sounds (think static noise, fan humming, water running, etc.) helps most people mask out the tinnitus so they can sleep better.