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."