My friend is a neuroscience Ph.D. and has done research into tDCS.
She says that tDCS devices sold to the public are certainly a scam, and they may even be unsafe. Don't buy them.
She also said that high-quality studies have pretty much uniformly debunked tDCS as having any sort of effect. The only studies with positive results were impossible to reproduce or didn't hold up with larger, better-controlled trials.
Whatever positve effect people are getting, they're likely feeling a placebo effect. My friend also often says that the most important factor in alertness (assuming at least a few hours of sleep) is motivation.
So someone who gets up to do a job they hate will believe they have less energy, even if they badly want to be awake. Likewise, someone getting up to do a job they love can hardly wait to get up, and they feel extremely alert, even with the same amount of rest.
