In sleep studies of (a) patients with the "fibrositis syndrome" and (b) healthy subjects undergoing stage 4 sleep deprivation, we observed in both groups the anomalous presence of alpha-rhythms in the non-rapid-eye-movement (NREM) sleep EEG. This phenomenon has been termed alpha-delta sleep. In the healthy subjects stage 4 deprivation was accompanied by the temporary appearance of muscoloskeletal and mood symptoms comparable to the symptoms seen chronically in the patients. It is suggested that the external arousing stimulus, which induced alpha-delta sleep in the subjects, is paralleled in the patients by an internal arousing mechanism. Such a mechanism, acting in competition with the NREM sleep system, would impair the presumed restorative function of NREM sleep and lead to the development of symptoms. It is proposed that the "fibrositis" symptom complex be considered a "non-restorative sleep syndrome". Evidence froms presented in support of the hypothesis that a disorder of serotonin metabolism serves as a basis for both the EEG sleep disturbance and the symptoms.