The effect of shifting an environmental factor
We can observe the effect of the light/dark cycle on plants and animals in the lab by shifting the lighting schedule of an organism's environment. For example, hamsters become active at dark every evening (possibly running on a wheel) and retire to their nests when it gets light every morning. Imagine that a hamster is kept on a normal schedule of day and night — a light over its cage is turned on from 6 a.m. to 6 p.m. and is left off from 6 p.m. to 6 a.m. If kept on this schedule for a week, the animal will follow a standard pattern for hamster life, starting to run at about 6 p.m. and going back to sleep at 6 a.m. If the lighting schedule is then changed so that the lights go on and off three hours later (light from 9 a.m. to 9 p.m.), the hamster will soon shift its own schedule to match.
Such shifts in timing can be more extreme. If the lighting schedule is shifted by 12 hours, both the animal and the plant will eventually become completely reversed in their activity cycles, beginning the day when they would normally be ending it and vice versa. Although, it may take as much as a week for its activity cycle to shift completely to the new schedule. People who work the "night shift" for a few weeks or more often experience this kind of complete reversal of the activity/rest cycle.
Delayed Response in Schedule Shift
Figure 3.2 shows the results of an experiment on sugar storage in the livers of over 600 mice over nine days. (The plotted points are given as a percentage of the average measurement for that day.) The solid line shows the average amount of glycogen (stored sugar) on day one, when the mice were all still on a schedule of lights-on from 6 a.m. to 6 p.m. After shifting the lighting schedule by 12 hours, to lights-on from 6 p.m. to 6 a.m., measurements were again taken. On day five, the cycle had shifted only about halfway to the new schedule. It was not until day nine that the cycle had shifted the full 12 hours.
You may have experienced such a delay in adjustment yourself if you have ever made a long-distance flight to a much different time zone. You may have felt a little "off” for as much as a week while your body's cycles were shifting their peak times to match your new sleeping and eating schedule. This phenomena, called Jet lag, is not the result of flying or of distance traveled; it occurs because your body needs time to adjust to a shifted sleeping and eating schedule when you arrive in a new time zone.
More evidence that the environment has only partial control over most biological cycles is found in the differences in the speed with which different cycles shift in the same individual. In one experiment, a man's schedule was shifted 12 hours. For a week before the shift and for a week afterward, measurements were made every few hours on many biological variables. How would each variable respond to the reversed schedule? The peak in excretion of phosphate by the kidneys began to occur earlier, and in four days had made a full 12-hour shift ahead. The peak for calcium excretion also shifted to earlier and earlier hours, but in four days had moved only eight of the 12 hours. On the other hand, the peak for blood pressure began to occur later, but in four days had shifted only four of the 12 hours. Sex hormone excretion also shifted later, but in four days had shifted only two hours. The 12-hour shift caused some of the man's biological cycles to shift earlier and some to shift later and all at different rates. Other cycles showed still different rates of adjustment to the shifted schedule. Only a few had completed the 12-hour schedule shift by the end of the week of measurements. Although changes in the environment exerted strong influence on the biological cycles, the cycles each responded differently to that influence.
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