It is interesting to note that not one problem from negative side effects has ever been reported in any published CES study. None of the patients has raided the fridge during the night and gained weight. None has complained of grogginess the next day. None has complained of headaches or a foggy feeling following treatment. Nor has CES been associated with increased suicide rates. When asked, CES patients have reported instead feeling more rested, more alert, and less tired following treatment.
One interesting clinical detail we learned early on is that patients who have not been sleeping well when they enter a study – many of them, by definition – sometimes make up for lost REM sleep during CES treatment and have the most vivid, most colorful dreams they have ever had. We learned to warn study participants of this in advance, since some earlier patients associated this with incipient schizophrenia or some other serious mental condition. Once alerted to the possibility they have always looked forward to the effect with real anticipation.
from A Summary Look at CES Studies of Cognitive Function, Ray B. Smith, Ph. D