The effects of weightlessness on the organism as a whole may be manifested by important changes in certain integrated behavioral patterns having an inherently rhythmic character. Modifications in basic behavioral patterns and performance may occur as disruptions of rhythmic physiological phenomena, which are themselves the end product of interrelated functional activity in a number of physiological systems, such as the neuroendocrine, cardiovascular, and central nervous systems. Measurements of interdependent components of biological rhythmicity are beginning to be analyzed by methods well established in physics—including correlation and spectral analyses, and phase modulation and variance in rhythmic processes. A wide variety of physiological functions can be treated as periodic variables in the analysis, including rhythmicities in cardiac output and blood pressure, respiration, brain waves, and the slower tides of appetite, and sleep-wakefulness. The importance of such investigations argues for their inclusion in forthcoming flight programs. Their experimental simplicity is an additional advantage. Biorhythms have been discussed in more detail in the section on "Environmental Biology." Effects of Weightlessness on the Cardiovascular System Earlobe oximetry, indirect measurements of blood flow and of blood pressure by finger plethysmography or impedance plethysmography, and ballistocardiographic techniques have potential application to manned space flight. Adaptation to prolonged exposure to weightlessness or to lunar gravity may cause difficulties when the astronaut is exposed again to reentry forces and terrestrial gravity. It is possible that these adaptive changes may thus produce unacceptable effects on performance or cause risk to life. It is important to obtain experimental evidence on this subject. It is common knowledge that following a stay in bed, dizziness, faintness, and weakness characterize arising, and that a feeling of general weakness may persist for several days. The phenomenon has been investigated in a number of laboratories. One approach has been to put healthy young subjects to bed, and even in extensive casts for periods of 2 or 3 weeks or more. Two major findings have emerged from these studies. First, a substantial adjustment in the blood circulatory system occurs, which is termed the "hypodynamic state." Second, there is a large decrease in the skeletal and muscle mass of the body.