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The Germans invented a word Zeitgeber which is useful in thinking about how the body’s circadian phases are entrained to the real world. Zeitgeber literally means time-giver. Zeitgebers are events and signals that tell the body what time it is.
Light is by far the most powerful zeitgeber – outdoor light from the Sun. (Indoor light is not nearly as bright.) Sunrise and sunset change with the seasons but there is always a dawn and a dusk and the level of light entering the retinal ganglion cells in the eyes affects the suprachiasmatic nucleus. The SCN is called the body’s master body clock for a reason. It signals release of melatonin, the hormone that circulates through the body and affects show-term daily changes.
Other physical environment zeitgebers include temperature and ambient noise level. Our minds and bodies know we are supposed to be up and around when it is warmer and things are hopping.
Social zeitgebers include other people yawning and going to bed. If you eat dinner at the same time every night your body might get used to that timepost and going to bed a certain number of hours later. If you watch a certain late night television show every weeknight before you go to bed, the precense of that programming on the TV screen can be a psychological zeitgeber and make you sleepier.
And the zeitgebers do result in you feeling sleepier or more alert. Many blind people have insomnia because they do not get the signal from daylight entraining their cycle to the environment.
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