Talk:Circadian rhythm/Archive 1

Previous discussions
What about blind people? What happens to their rhythm?

There should also be some discussion of the rhythm in absence of stimuli. There was a study done, though I forget where, which roughly showed that a great deal of mammals naturally act on a cycle of 26-28 hours in the absence of a day-to-night cycle and thrive there stably. I can personally attest to this, in fact, as I pay little attention to the sun and by my own schedule I usually prefer to sleep 12 hours and be awake 15 or so, creating schedule problems.

Blind people will be exactly the same as everyone else. The perception of light is not the same as seeing light. Even if there is a problem with the eyes the body is still able to perceive light, and it knows when it is day and when it is night. The perception of light is simply the input stimulus for the system. From there, a cascade of hormones affect an even greater number of genes, ultimatly setting the clock.

For your second point, there have been several studies done in humans, other animals and especially plants, all concerning circadian rhythms in the absence of light. Currently, the evidence suggests that there is an intrinsic cycling that does occur. I work mostly in plants, and I know for plants that once a cycle is set, moving a plant into total darkness, initially has little effect. The cycling becomes programmed and takes a long time to reset.--Doucher 18:03, Apr 23, 2005 (UTC)


 * This seems to go directly against the facts stated in the article that say circadian perception of light comes from the retina.


 * ...and plants don't have retinas or ganglion cells ... Paragraph deleted by Jclerman 21:17, 1 October 2005 (UTC)


 * And I think I've read stuff saying some blind people do have problems with their circadian rythm. Anyhow, this topic should probably either be treated (as it is relevant as long as references to the retina are made in the article), or the article changed (if it is wrong saying what it says). LjL 00:32, 4 May 2005 (UTC)


 * Blind doesn't mean "no photoreceptors", blind just means "pathway between eyes and visual cortex has been interrupted". The retinohypothalamic tract is distinct from the much larger visual pathway. The two paths diverge at the optic chiasm. Any blindness resulting from a post-chiasmatic injury would not affect light transmission to the clock.


 * There are also photoreceptors in the retina that do not participate in the visual pathway; damage to the visual photoreceptors that spares these will not affect the light-resetting ability of the retina acting on the SCN.


 * Finally, don't forget that light is the most common entrainment signal, but it isn't the only one. A blind person with an alarm clock set to 8 AM will be every bit as entrained to the daily light cycle as you or I, provided that the clock maintains accurate time.


 * As for your second point, well, go back and look at the research. Endogenous periods are all over the place, but tend to be slightly less or slightly more than 24 hours, not predominantly 26 or greater. --DrNixon 06:24, 30 January 2006 (UTC)

SCN and free running

 * I made a few changes that might help with these issues... added info that the cells of the SCN maintain their own rhythm in isolation (cell culture).... adding support to the "free running" phenomenon. The SCN is influenced..i.e..entrained by light... but has its own rhythm. Blind people essentially free run unless they have external cues to help entrain them. A rigid schedule can be as effective as photic entrainment.

MrSandman 29 June 2005 19:37 (UTC)

25 hr cycles
I remembered while I was reading this article one of the few things I picked up from a psychology class - human circadian rhythms are 25 hours rather than the 24 the article claims. Numerous sources easily found through a search engine will confirm this - http://www.sleepdisorderchannel.net/jetlag/ is just one of many. I've edited in a short bit about this to reflect this somewhat counterintuitive fact. 69.92.141.242 08:58, 26 September 2005 (UTC)


 * It's not counterintuitive for me. Diurnal animals have cycles longer than 24 hours and nocturnal animals shorter than 24 hours. Why did natural selection result in such a difference with the 24 hrs day/night succesion is the question to be asked. The answer lies in the analysis of the response of oscillatory systems to periodic entrainments. Jclerman 20:55, 1 October 2005 (UTC)


 * The article states "Free-running rhythms of diurnal animals are close to 25 hours." Studies the last 10 years or so have debunked this (w/regard to humans).  The average for human adults, young and older, is now accepted to be 24 hours and 11 minutes.  --nbm, 24 May 2006


 * I'll assume you know what you're talking about, nbm, but the way you've edited it now doesn't make sense. I'm changing it. 163.1.143.112 (talk) 01:42, 24 March 2008 (UTC)


 * That section had indeed become mangled; thank you for pointing it out. Better now?  --Hordaland (talk) 03:31, 24 March 2008 (UTC)

Plants are able to adapt to any light regime ...
Please give a source for the temporarily deleted paragraph:
 * Plants are able to adapt to any light regime a laboratory can simulate, including ones that don't exist in the plant's natural environment, such as a 5 hour light cycle or an 82 hour light cycle. However, a plant's circadian rhythmns do not come solely from it's environment. A plant with no environmental input (a plant kept in the dark) will revert to it's genetically programmed 24 hour cycle for opening and closing flowers or moving it's stem and leaves. Jclerman 12:49, 19 November 2005 (UTC)

The paragraph above has been corrected to reflect the idea of a genetically preset circadian period in relation to relatively small amount of published work that addresses this question.

In general, this whole page could do with more references to published work.

October 6, 2006 131.111.8.96

Rapid reaction by the SCN
"The SCN does not appear to be able to react rapidly to changes in the light/dark cues." I think that 1) this needs a source, and 2) it is meaningless anyway, as long as "rapidly" is not defined. --Beth (who doesn't really know how to do this kind of thing yet - will try to improve ;-))


 * That's actually a completely false statement. The SCN receives direct glutamatergic input from the retina and can entrain to light input during the subjective night due to this pathway. This entrainment occurs within 5 minutes (CREB phosphorylation) or longer (~30 minutes for PER1 I recall) depending on what exactly is meant by "react". If no one objects, I'll make the appropriate correction here. Dpryan 23:44, 12 July 2006 (UTC)dpryan

Plant Circadian Rhythms?
I'm very curious about your mention of circadian rhythms in plants. There seemed to be very little detail available. With animals, and even animal cells, there's a noticeable difference in behavior. Specific organs secrete different hormones and such. What exactly is the basis of circadian rhythms in plantlife that can't be explained by the organism immediately reacting to stimuli (like sunlight)? Would the plant behave according to a similar schedule even if sunlight were blocked?

I was a little thrown by the last statement: "Circadian rhythms are thought to have evolved completely independently in cyanobacteria, animals, plants and fungi." So, Circadian Rhythms have evolved independantly of life? No, this statement means that the molecular circadian clock in each of these kingdoms has separate evolutionary origins.

Have there been any studies done on how the day/night cycle affects single-cell organisms? For example, are there nocturnal bacteria?

-Thanks 24.126.178.118


 * These rhythms were initially discovered in the movement of plant leaves in the 1700s by the French astronomer Jean-Jacques d'Ortous de Mairan who observed them in the dark. For a description of circadian rhythms in plants by de Mairan, Linnaeus, and Darwin see The formal study of biological temporal rhythms (such as daily, weekly, seasonal, etc.) is called chronobiology. Entire laboratories are dedicated to research and to commercially produce plants in a variety of entraining photoperiods. Jclerman 09:26, 8 January 2006 (UTC)