Ultradian and Infradian Rhythms

Although most of us are not aware of them, environmental rhythms abound on Earth. As exemplified in Table 4.1 , there are rhythmic oscillations in our environment that range in period from a few femtoseconds (10 –15 seconds) to tens of thousands of years. Some of the rhythms are man-made, such as 50 or 60 Hz alternating electric current and the 7-day week, whereas others are created by the Earth, the Moon, and the Sun. For instance, ocean tides are caused primarily by the Moon’s gravitational attraction and secondarily by the Sun’s gravitational attraction. 1 With a few exceptions, the interval between tides corresponds to half a “lunar day”—or 12 hours and 25 minutes ( Figure 4.1 ). One exception is the U.S. coast facing the Gulf of Mexico, where most of the time there is only one tidal cycle per day—or, more exactly, per 24 hours and 50 minutes.