The study investigates how the human body composition (BC) changes as a function of the day-night cycle. The BC was investigated using bioelectrical impedance analysis (BIA) of 10 clinically healthy subjects (CHS), monitored in supine position (readings at 2-h intervals), avoiding mealtimes, dietary abuses, and bladder and intestinal retention. Time series data were analyzed for their temporal characteristics and circadian rhythm (CR). All the variables of BC (lean body mass, fat body mass, body cell mass, total body water, intracellular and extracellular body water, sodium and potassium exchangeable pool) showed a within-day variability with nighttime crests. Such an oscillatory synchronism corroborates the hypothesis that the rest time plays a fundamental role, via its anabolic effects, in conferring the nocturnal phase to the CR of the human BC.