Nightly restlessness in patients with Alzheimer's disease (AD) is probably due to a disorder of circadian rhythms. Transcutaneous electrical nerve stimulation (TENS) was previously reported to increase the strength of coupling of the circadian rest activity rhythm to Zeitgebers in early stage Alzheimer's disease (AD) patients. It was investigated in the present study whether TENS could also improve the rest activity rhythm of patients in a midstage. Sixteen patients who met the NINCDS ADRDA criteria for probable AD, and the stage 6 criteria of the Global Deterioration Scale were treated with TENS or placebo. Rest activity rhythm was assessed using actigraphy. Compared to the control group, stimulated patients showed an improvement in the rest activity rhythm of similar magnitude as observed previously in patients in an early stage. It is concluded that TENS increased the coupling between the rest activity rhythm and supposedly stable Zeitgebers in an advanced stage of AD.