58. Gender differences in sleep

11 September 2010 at 17:17 | Posted in Circadian rhythm | 2 Comments
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I’ve earlier seen hints that there are differences in men’s and women’s sleep timing.  Now a new study confirms that and has also found differences in the quantity of melatonin secretion and in the daily temperature amplitude.

The study participants were normal sleepers:  28 women and 28 men, ages 18-30, matched in pairs for age, habitual bedtime, habitual wake time and MEQ-results.  Under strictly controlled conditions, so-called constant routine, their core body temperatures and melatonin levels were measured.

The women reached higher levels of melatonin in the blood.

The men had a greater amplitude in body temperature throughout the day and night.

The illustration shows the significant differences in sleep timing between women and men, on average.  In each of the 28 matched pairs of participants, significant differences were found between the women and the men with regard to the intervals

  • between DLMOn and bedtime,
  • between wake time and DLMOff, and
  • between temperature minimum and wake time.

The women were sleeping and waking at the same clock time, but at a later biological time than the men.


  • MEQ = the Morningness-Eveningness Questionnaire by Östberg and Horne
  • DLMOn = Dim Light Melatonin Onset
  • DLMOff = Dim Light Melatonin Offset (Here, based on blood level, not offset of synthesis.)


Reference:  Cain, Sean W., Christopher F. Dennison, Jamie M. Zeitzer, Aaron M. Guzik, Sat Bir S. Khalsa, Nayantara Santhi, Martin W. Schoen, Charles A. Czeisler and Jeanne F.  Duffy.  Sex Differences in Phase Angle of Entrainment and Melatonin Amplitude in Humans.  Journal of Biological Rhythms 2010 25: 288.  DOI: 10.1177/0748730410374943


Next post: Coming soon


53. Light therapy: white, blue or maybe green?

30 May 2010 at 14:00 | Posted in Circadian rhythm | 3 Comments
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It has long been known that light and dark affect the daily as well as seasonal rhythms of living things.  Early in the 1900s it was assumed that humans’ daily rhythms were less affected than those of “lower” beings, but that attitude was proved wrong.

In the 1980s it was noted that some totally blind people entrained perfectly to the 24-hour cycle while many did not.  Until some time in the 1990s, it was not known if entrainment occurred by light to the eyes or to the skin.  One research team claimed to have proven that light to the backs of the knees effected entrainment, but neither they nor other researchers could duplicate those results, which later were withdrawn.

It is now clear that we and other animals entrain primarily by light to the eyes, though secondary cues such as social activity, feeding times etc. also play a role.  It is also clear that our eyes contain not only rods and cones for vision but also the recently discovered light sensitive ganglion cells for the entrainment of circadian rhythms.  The light sensitive pigment in these cells is melanopsin.

There it stands, but there are always new questions to be answered.  One is: 

Do the light sensitive ganglion cells contribute to vision,

and do the rods and/or cones contribute to

the non-visual effects of light?

If the ganglion cells’ photosensitivity contributes to vision at all, it is very minimally.  But a study* published this month by well-known researchers suggests that the cones do affect entrainment, with variations dependent on the timing and intensity of the light.  The practical implications include the question of whether the use of blue-blocking goggles in the evening, so-called “dark therapy”, really does allow normal flow of melatonin as intended.  [An aside: mightn’t there be a more direct way to find this out?]   In addition, the jury is still out on what color light – white, blue or perhaps green – is best to use in light therapy.

In this study in Boston, more than 50 human subjects each spent 9 days in an laboratory environment free of time cues.  Semi-recumbent, totally confused about the time of day, and with their melatonin rhythms individually determined, half of them were exposed to blue (460nm) light and half to green (555nm) for 6.5 hours starting shortly after their own melatonin secretion started. 

It is well-known that blue light (including about 460 to 482nm) excites melanopsin leading to the suppression of  melatonin.  The question here is whether the green light may have similar or equal effects. Green was chosen because the human visual system is most excitable at green (555nm).

In the figure, A and B are two subjects.  Each person’s normal pattern of melatonin secretion is repeated on the left and on the right in black.  On the left, blue light acutely suppresses A’s melatonin, while green light only delays B’s for a good hour.  The next night, as we see on the right, the pattern of melatonin secretion has been phase-shifted by the previous night’s light exposure about equally for the two subjects (horizontal red line).  From previous knowledge, one would have expected an appreciably greater delay after the blue light than the green.

The authors write: 

“Our data … raise the possibility that activation of cone photoreceptors in the late evening by relatively low-illuminance light sources, such as liquid crystal display monitors, table lamps, and dimmable lamps, may delay the circadian clock and therefore contribute to the high prevalence of delayed sleep phase disorder.” [My emphasis.]


“[B]locking short-wavelength light with blue-blocking goggles may not always be effective in preventing undesired circadian responses based on our finding that longer-wavelength light is able to induce robust phase-shift responses.”


“Our findings have implications for the development and optimization of light therapies for a number of disorders, including circadian rhythm sleep disorders.…”

* Spectral Responses of the Human Circadian System

Depend on the Irradiance and Duration of Exposure to Light.

Joshua J. Gooley, Shantha M. W. Rajaratnam, George C. Brainard,

Richard E. Kronauer, Charles A. Czeisler and Steven W. Lockley

Science Translational Medicine, 12 May 2010 

See also these older posts: 

xliii. Blindfolding the blind   

xliv. Rods and cones and the “new” ipRGC 

 (posted by D )


Next post:  #54. Take a Nap!



xx. Entrainment

14 December 2005 at 06:45 | Posted in Circadian rhythm | 3 Comments
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“To entrain”, according to my old Webster’s, means to board a train.  I need a new dictionary.
When biologists say that an organism is entrained, they mean that one or more of its circadian (about a day) periods is reset to coincide with cues from the environment.  In the laboratory they can entrain animals or other organisms to various periods, within limits.  Light and darkness are the most oft-used cues; others are physical and social activities, temperature and drugs or chemicals.
When a healthy entrained human subject is chosen for a study, this means that he (seldom she) at least once daily resets his central pacemaker to conform to the 24-hour period of the natural light/dark cycle.  As almost all of us do.
When biological clocks are not entrained, as when, for example, existing in constant darkness without time cues, they are said to be freerunning.  The average freerunning period for human adults is, as noted, 24 hours and 11 minutes.  Our own clock, as well as that of rodents, fruit flies and others, “tells time” in order to be able to program activities at appropriate times.  Its key function is “to provide an internal estimate of the external local time” (Johnson, Elliott and Foster, 2003).
As far as I can see, the researchers have plenty left to do.  There are unanswered questions about effects of different wave lengths, duration and intensity of light exposure as well as gradual changes in light intensity, how the light-sensitive cells “work” and whether their sensitivity varies throughout the day and night.  And, of course, I’m waiting for some explanation for Delayed Sleep-Phase:  why/how one can adjust to waking at the same time every day, just not the right time every day!
Next post: xxi. Is it life-long?

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