It is good that you are concerned for their health.
If they were licking Dirty
sheets, there might be a slim chance that they were craving some kind of mineral, but since New
sheets are setting off these cycles, it more certainly started out as a desire to set down scent on the sheets that have been washed clean of their body odors. But just to be sure, check their teeth and gums to make sure there are not dirty spots or loose teeth that might also leave a strong bacterial taste behind.
Licking obsessions are very common in pet dogs. Some dogs lick their People, some lick a favorite Pillow or Cushion, the Rug, the Bed, etc.. Psychological anxiety (usually brought on by boredom) can worsen an oral fixation. Two dogs with the same interest in fabric can get trapped in a feedback loop with each other.
This type of behavior starts out innocently enough...a dog lies comfortably on the bed and begins to groom its feet and toes. The tongue slips off and brushes the sheet. This is a New and Interesting sensation (depending on how sensitive the tongue is and how bored the dog might be at the time). Salivary proteins bind to the fabric and keep a dog's attention and licking going for some period of time. Once it has reached the "hours" level, we can safely assume that an Obsession and/or Compulsion has been established in the brain.
This is an article about all the reasons dogs might do this:Read Article Here
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