If you've ever stayed up during hours when your body craves sleep, you've played with your body's natural "circadian rhythms" (circadian from the Latin "circa" = about, "dia" = a day). Your rhythms are determined by your brain's hypothalamus and help keep an internal clock by using the sun, dark and other cues (such as eating and sleeping at the same time). Aside from determining your sleep cycles, the circadian rhythm helps regulate hormones and tells you when to use the restroom or grab some food.
When you don't commit to a regular, full sleep period, you fall prey to problems. One of the most common issues is shift work sleep disorder, which comes from working during a time when we're "set" to sleep. Symptoms of shift work disorder include excessive sleepiness, insomnia, headaches, lack of energy, increased accidents and mood problems. Another common problem that comes from confusing your circadian rhythms is jet lag.
For example, if you take the red-eye from NYC to Paris and arrives in Paris at 7 a.m. (light, awake) your body clock is confused since it expects that it is 1 a.m. (dark, asleep). You then expose yourself to sunlight immediately upon landing, thereby exacerbating the problem since the brain thinks it should be dark.
Symptoms of jet lag sleep (fatigue, insomnia, grogginess, irritability, mild depression, intestinal upset) can take upwards of two weeks to resolve until the circadian clock adjusts to the new time zone. The basic rule of thumb is that it takes approximately one day for the body to adjust to each time zone crossed. Not everyone who works shifts or travels across time zones has problems adjusting, but many do.
Although the above examples are extreme versions of confusing your body clock by being awake regularly during times when you need to sleep (or trying to sleep when you biologically want to be awake), a new term has been coined in the sleep medicine literature by a group of researchers in Europe—social jet lag. Til Rotenberg and his colleagues suggest that our modern society creates a mismatch during the demands of a) what needs to be done and when it needs to be done versus b) our programmed circadian rhythms. What happens is the following: You keep a regular sleep-wake pattern on work-nights (e.g. bed at 11 p.m., wake at 7 a.m.), but on nights when you don't need to work, you usually don't set the alarm clock and then sleep later and often go to bed later to socialize. You end up getting more sleep on the weekends and adjust to a later bedtime and a later wake time.
Switching sleep schedules feels the same way to our bodies as does switching time zones—the only difference is that you don't need to fly anywhere to get the same detrimental effects. Many people prefer to keep a later bed and wake time (e.g. 12-1 a.m. to bed, 8-9 a.m. wake), but get less sleep during the week, because they wake much earlier than they would otherwise prefer to get ready for work.
In a 2012 issue of Current Biology, Rottenberg and colleagues estimate that nearly two-thirds of the population has social jet lag! The Rottenberg group linked it to higher rates of obesity—less sleep and an out-of-sync circadian rhythms can slow one's metabolism and lead to weight gain. You're eating at times when our bodies aren't physiologically programmed to digest food. In addition, it has also been suggested that social jet lag may lead to similar problems seen in those who have shift work sleep disorder, including diabetes and increased cancer risk. More research is needed in this field to see if those theories are true.
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