Throughout history man has observed such natural cycles as the rising and setting of the sun, the ebb and flow of the ocean tides, and the changes of the seasons. The 1 nature of these events provided people with a measure of 2 about things to come and give them reason to reflect on their past and plan for their future. The regularity of such social activities as work hours, meal times, and holiday schedules 3 the extent to which human activity itself is organized in cycles. Recently, 4 effort has been spent investigating the effect of various biological rhythms in the human body. 5 , interest in these rhythms at least in part results from man's basic desire to predict future behavior. The human body 6 a number of repeatable rhythms: heartbeats, breathing rates, brain waves and other physiological processes within the human machine. Scientists studying rhythms in humans have measured sleep patterns and attention 7 over periods of several days, and have 8 the fact that the cycles exist and exert effects on physiological and sociological behavior. A common example is jet lag. A person's traveling in the east-west direction across time zones 9 his sleep cycles—it may take several days to return to the 10 cycle or to readjust to a new one.A) indicates E) considerable I) Certainly M) securityB) demonstrated F) upsets J) Thankfully N) exhibitsC) verifying 6) disappoints K) spans O) limitsD) normal H) periodic L) gorgeous