The quoted study is poor quality, and is not sufficient to accept the claims.
Earthing in general is pseudo-scientific nonsense.
The study that supports the claims by Stevenson is The Biologic Effects of Grounding the Human Body During Sleep as Measured by Cortisol Levels and Subjective Reporting of Sleep, Pain, and Stress, which was published in the Journal of Alternative and Complementary Medicine. It is known for publishing poor quality woo articles. Quackwatch list it under "Journals (Fundamentally Flawed)" in their list of Nonrecommended Periodicals.
But that just means our skeptical hackles should be raised. It doesn't mean an individual article might not be high-quality.
In 2007, noted skeptic Orac (a.k.a. Dr David Gorski, took a look at the experiment described in the article. (Unfortunately, at the time of writing, some of the images are now broken links)
He identified some major flaws:
The experiment wasn't blinded.
Given the lack of blinding in this study, of patients or investigators, it’s impossible to tell whether the subjective reports by patients of improved sleep and feelings of well-being are anything more than the placebo effect.
The measurements were too noisy to draw a conclusion
I have to wonder how they collected saliva samples without waking up the patients. Did they in any case, saliva cortisol levels, although they correlate with blood cortisol levels, can be affected by drugs and/or variations in the levels of different plasma proteins. Worse, they compared single day measurements, which are prone to large day-to-day variations:
Single day assessments are very weak approaches to this problem since measures are affected by many day-to-day variations, and this is especially difficult when the shape of the rhythm is of interest, since this seems rather sensitive to the influence of stress.
The statistical analysis was insufficient
Sorry, guys, but the “eyeball test” usually isn’t enough, especially when you’re using only 12 patients and especially when you didn’t bother to do a few repeat measures.
In other words, this study is, in essence, meaningless.
Brian Dunning at SkepticBlog reached a similar conclusion.
Earthing Therapy is Pseudoscience
The field is groundless. [No, I won't apologise for that pun.]
Biologist and noted skeptic, Pharyngula (a.k.a. PZ Myers) summed Earthing up as "classic crank pseudoscience"
- Sweeping claims of incredible health benefits from one simple mechanism.
- All you need to “earth” yourself is a grounded pad—which they’ll sell you for the low, low price of $349.95.
- Grain-of-truth biology (free radicals can cause cellular damage) coupled to extravagant and silly claims (the infinite flow of electrons from the earth will stop free radicals from hurting you).
- Lots of repetitive, long-winded gobbledygook to justify freaky ideas.
- Fond reminiscences of the good ol’ days, when people were always grounded and never, ever got sick…you know, like in the 19th century.
Another noted skeptic, Dr Steven Novella looks at some of the basic claims on the Neurologica blog:
Completely blowing the physics aside, Ober and his accomplices then go on to butcher biology. The earthing site claims, as in the quote above, that inflammation is caused by free radicals. This is simply not true. The relationship between free radicals and inflammation is a complex one. It is probably more true to say that inflammation (which is caused by specific cells and proteins produced in an inflammatory response) causes the production of free radicals, which are used to cause cell damage. Free radicals are part of the weapons the immune system uses to damage invading organisms, for example. This also causes damage to host tissue as a necessary byproduct. Abnormal inflammation, of course, can primarily cause tissue damage.
Further, reducing free radicals is not a panacea. Free radicals are part of normal physiology and are used not only as part of the necessary function of the immune system but in many regulatory systems. Suppressing free radicals may therefore cause more harm than good.
Every link in the earthing chain of argument is therefore wrong. It is little more than free associating with sciencey terms (i.e., making shit up).
Earthing is a true pseudoscience in that it claims to be scientific. [...] The studies are typical of the kind of worthless studies designed to generate false positives – the kind of “in house” studies that companies sometimes use so that they can claim their products are “clinically proven.” Reading through the individual studies [...] you can see that they are all small pilot or preliminary studies with atrocious methodology. They are little more than documenting placebo effects, subjective findings, and anomaly hunting.