386 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
electrons sweeping in orbits about positively charged nuclei, to our 
solar system of planets, including the earth, revolving in their orbits 
about the sun, the observable, relative stability of nature is dependent 
upon such rhythmic or periodic changes. 
The external physical environment of living organisms displays 
well-known rhythmic changes. The rotation of the earth on its axis 
relative to the sun provides the daily 24-hour period with its extensive 
changes in light, temperature, and humidity. The rotation of the 
earth relative to the moon provides our lunar-day periods of 24 hours 
and 50 minutes. The lunar-day fluctuations in light and temperature 
are, however, feeble compared with those of the solar day. These two 
daily rhythms cooperate with the orbiting of the moon about the earth 
to provide monthly cycles of 2914 days, the period separating two 
consecutive times of synchronization of solar and lunar noons. Both 
sun and moon are responsible for producing tides in the oceans and 
atmosphere of the earth. The sun is the more influential of the two 
in the case of the atmospheric tides, and the moon the more influential 
for the oceanic ones. The solar and lunar tides of the atmosphere are 
associated with more subtle physical changes such as barometric pres- 
sure, cosmic radiation, gravity, magnetic field, and atmospheric elec- 
trical potential. The earth also revolves about the sun with its axis 
oriented in such a manner as to yield the annual rhythm of the seasons. 
Finally, the sun itself rotates on its own axis with an average 27-day 
period, and since the sun’s surface is not uniform, an additional 
rhythmic change with a 27-day frequency is imposed upon the surface 
of the earth. 
The physical environment of living things is truly a composite of 
rhythms. 
It might be presumed, therefore, that living things, as complex, 
delicately balanced, physico-chemical entities lying at a level between 
atom and solar system, persist as stable beings only through some 
kinds of comparable rhythmic fluctuations about mean values. In 
fact we do not need to go beyond our own bodies to be impressed with 
the existence of such rhythms. Man has daily cycles of many processes 
such as wakefulness, blood-sugar concentration, blood-cell count, and 
body temperature. For example, our body temperature tends to be 
lowest, other factors equal, about 5 to 6 in the morning and highest 
about 5 to 6 in the evening. Another familiar cycle is the reproductive, 
which follows the lunar month. 
Very widespread among the animal and plant kingdoms are daily, 
lunar-tidal, lunar monthly, and annual cycles of innumerable activi- 
ties; and where biologists have sought them, 27-day cycles have been 
observed as well. It can be said without equivocation that rhythmic- 
ity is the rule in living things. These rhythms are often highly useful 
