This is the 2007 version. Click here for the 2017 chapter 03 table of contents.

Advancing the Daily Rhythm

The circadian rhythm is 24 hours, but the body's internal clock produces a cycle of 25 hours, when left to its own devices. Evidently our daily rhythm is driven by the slightly faster circadian or 24 hour rhythm. The light/dark cycle acts as a pacemaker. Without the pacemaker, we easily adopt a longer rhythm.

What causes "Sunday night insomnia"?

Due to the 25-hour natural rhythm, humans easily shift their sleep schedule forward (getting up later and later) but have trouble shifting it backward. This produces a phenomenon known as Sunday night insomnia. If you stay up late and sleep late over the weekend, you can easily adjust your body rhythm forward one or two hours a day. Over the weekend you can shift your internal clock forward up to four hours, so midnight feels like 8 p.m. when you try to go to sleep Sunday night.

Insomnia is cured more easily by moving the rhythm forward instead of backward. An insomniac who normally stays up until 3 a.m. might be asked to stay up until 5 a.m. before going to bed. The next day, bedtime is 7 a.m., the next day 9 a.m. Eventually such a person can work around to a normal bedtime. This works in some cases when everything else fails.

Czeisler, Moore-Ede, and Coleman (1982) applied the findings about the ease of "shifting forward" to solve problems at the Great Salt Lake Minerals and Chemicals Corporation in Ogden, Utah. Workers at this plant were bothered by weekly shift rotations that required them to change from night to evening and then to a day schedule. Czeisler and the other researchers realized this backward shift was fighting the natural body rhythms of the workers. They tried reversing this pattern. They had workers move from day to evening shift, then from evening to night. Instead of getting up earlier for a new shift, the workers stayed up later. They liked this better. Job satisfaction ratings improved, employee turnover at the plant decreased, night production improved, and health complaints went down.

How did Czeisler use this knowledge to solve a problem involving shift workers?

For similar reasons, it is easier to recover from jet lag—the feeling of being on a different time schedule than the rest of the world—when going from east to west, rather than the other direction. When you fly west, you get to stay up later...unless you fly more than halfway around the world! Then you may end up on an earlier schedule, as far as your body is concerned.


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