Yes, we recommend fitting the Earthing Sheet underneath your current bed sheet providing that any covering bed sheet is made from natural materials such as cotton or bamboo.
The effectiveness of this can be verified with a multimeter device which demonstrates a strong Earth connection is still achieved when the Earthing sheet is fitted underneath a cotton bed sheet, even with an additional layer of clothes/pyjamas before reaching the skin level. In addition, as you lie on the covering sheet, perspiration and environmental humidity will hydrate your sheet which further strengthens the Earth connection.
When using the Earthing sheet underneath a partially synthetic covering sheet, such as cotton with polyester, in general, the Earth connection is only slightly reduced and still provides sufficient Earthing for the body. However, if you wish to test this yourself to ensure the Earth connection is not being blocked, we are happy to recommend a brand of conductivity tester.