It's often considered an old wives tale these days, but I've always known teeth grinding (at night) to be associated with intestinal worms - it's easily treated, so it might be worth checking if she has any other symptoms.
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# Constipation or diarrhoea
# Drooling when asleep
# Fatigue and general feeling of unwell
# Feeling tired on waking up
# Flatulence
# Food sensitivities
# Fuzzy thinking
# Gastrointestinal upsets
# Grinding teeth at night
# Intestinal cramps
# Itchy ears, nose and anus
# Lethargy
# Numb hands
# Skin complaints (i.e. rashes)
# Vaginal irritation
# Unexplained weight gain or loss