Skin is poorly conductive. When through-skin transmission is desired, steps must be taken to improve conductivity or increase surface area, such as use of
abrasive and conductive gels,
needles, or
conductive pads.
Also, static charges may accumulate on regular bedsheets and clothes.
As such, a grounding strap may not be electrophysiologically equivalent to a large sheet.
Ambient temperature, humidity, hydration level may affect results.
Changes may take more than one night take effect.
Results may be measureable but imperceivable, eg minor changes in blood pressure, heart rate.
A washout period may be required to allow for effects to disippate.
Good luck!