Monday, April 2, 2012 CC-BY-NC
Circadian rhythms and neuroendocrine regulation

Maintainer: admin

1Characteristics and classification

  • rhythm: if you could draw a sinusoidal wave for it
  • types
    • ultradian rhythm : <20hrs (ex: heart beat, neuron firing)
    • circaidan rhythm: between 20 and 28 hrs (ex: ~ a day) <-- our focus
    • infradian rhythm : >28hrs (ex: change of fertility, migration, etc)
  • example: Three different hormonal rhythms in sheep##
    • prolactin rhythm is in infradian rhythms (measure of weeks)
    • melatonin is in circadian : high at night
    • LH is ultradian
  • example: in human
    • body temp is high in day and low at night
    • cortisol is high in early day and decrease throughout the day.
      • somatostatin is the opposite since it downregulate cortisol (high when cortisol level is low.. like around sleeping time)
  • time isolation experiment in caves and Jurgen Aschoff's bunker:
    • Michel Siffre, a French speleologist, isolate himself in the caves to evade environmental clues
    • both cases show that though circadian rhythms are a bit off from 24hr, we still have endogenous rhythms.
    • even w/o light, we still have rhythms cuz we're structure that way (that binds us to our mother earth!)


  • melatonin levels can be masked by light
    • exposing sheep with bright light in their dark cycle induce a sharp dive of melatonin level.
  • core body temp. can be masked by sleep
    • napping during the day induces a drop in core body temperature, which is supposed to be high during the day and low at night.
  • cortisol rhythm can be masked by stress
  • conclusion: rhythmic physiological phenomena can be masked and can be entrained by environmental clues.
    • Therefore, to assess circadian rhythm, we must remove factors that can mask rhythms
      • i.e. constant routine protocol (see next). everyday is the same. no stress, etc
    • also forced desyncrhony protocols
      • no window, no clock, subjects are told to sleep and wake up at certain time.

2.1Constant routine protocol

  • constant routine protocol : a way to keep environment constant
    • sustained wakefullness (ex: for 40 hrs straight)
    • semi-recumbent posture + limited activity (subjects are told to not completely lie down nor standing)
    • dim light : to avoid masking melatonin level.
    • hourly snacks (constantly feeding) : to avoid masking by food-intake schedule.
    • some effects that are seen:
      • cortisol and melatonin amp and period remain almost the same --> so their circadian rhythms are very endogenous
      • growth hormone and prolactin amp flattens, since both of these have a lot to do with waking/sleeping cycle.
      • TSH changes phases.
    • when a subject is forced to live on a LD(light-dark) cycle of 28 hours
      • circadian cycle becomes uncoupled from sleep-wake cycle cuz human circadian cycle cannot go over 26 hr.


  • Human's endogenous rhythm is about 24 hr and 14 minutes (no wonder that extra 10 min in the morning matters so much...)
  • for rodents, their cycle is about 22 hr. they're also nocturnal.
  • Rodents' motor activity is recorded by turns of wheels.
    • each horizontal line is one day of recording.
    • We can see that in LD cycle, rodents start running when it's dark.
    • when environmental clue is removed (when it's DD, 24 hour darkness), rodents start running ~2 hrs earlier each day.
    • this help us determine the period of the endogenous cycle of rodents.
    • autograms are also useful tool to see how mutations in certain genes (esp clock genes, clock-controlled gene) affect endogenous rhythm.