ix. Melatonin and the effect of light

26 November 2005 at 16:49 | Posted in Circadian rhythm | Leave a comment
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Oops, sorry.  Got a little ahead of myself there.  Melatonin should have been explained before that more technical stuff.
 
Melatonin is a hormone produced and secreted by the pineal, a tiny, midbrain gland which doesn’t seem to have any other tasks.
 
The pineal takes its orders from a nearby organ which Mary Poppins surely should have sung about, the suprachiasmatic nuclei.  However you choose to pronounce that, it’s a whole dance tune in itself.  The SCN (for short) is our central biological clock.  If totally isolated from outside cues about time, it will keep running on its own “almost a day” (= circadian) cycles virtually indefinitely.  Cells from mammalian SCN do so, in fact, in a dish.  
 
The body clock needs to be reset daily.  The SCN receives (or receive, if you prefer) information from the retina directly along a pathway called the retino-hypothalamic tract.  And yes, the SCN is located in the hypothalamus, in the brain. 
 
The vital information which the SCN processes, is sent to it by special light-sensitive cells in the retina in the back of the eye.  This information about light received at the retina has nothing to do with vision.  It tells our system when day and night are.  There can be other cues as well, such as mealtimes and activities, but light is by far the strongest.  
 
The rhythms of core body temperature (CBT) and melatonin secretion, at the least, need to be coordinated with each other and with the cycle of light/dark to allow us to fill our need for one long sleep-session, preferably at night.  
 
The level of melatonin in the blood during the (subjective) day is (or should be) near zero.  As CBT falls during an individual’s night, then starts rising again about two hours before wake-up time, our melatonin level rises during the first half of the night, to drop off during the second.  However, the two curves are not mirror images of one another.  The melatonin level rises quite sharply and then stays on a plateau.  It starts falling a couple of hours before the CBT bottoms out.  Here’s how this is shown in post no. xiii:  

 
 1133139135-hr-29  

Bright light banishes melatonin from the blood and stops / delays the secretion of it.  

A person with a normal circadian system who cooperates with nature’s signals will sleep at the same time every night.  The schedule might look like this:

  • 8 or 9 p.m., melatonin secretion starts, perhaps not measurable yet (DLMO, Dim Light Melatonin Onset)
  • 10 p.m., calming down, lights are dim
  • 10:30, feet are hot, person feels sleepy
  • 11 p.m., asleep
  • 3 a.m., melatonin level starts receding
  • 5 a.m., core body temperature minimum
  • 7 a.m., wake up, and light exposure banishes the remainder of the melatonin 

We all know people who’d rather sleep 10 p.m. to 6 a.m. or midnight to 8 a.m.  Those schedules are within the normal range.  

When I was a kid, medicine and science thought that the normal range should be achievable for everyone.  It is not.

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Next:  x. Other animals

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