Rhythmic changes in physiology and behaviour within a 24 h
period occur in living organisms on earth to meet the challenges
associated with the daily changes in the external environment. The
circadian pacemaker responsible for the temporal internal
organisation and the generation of endogenous rhythms of
approximately 24 h is located in the hypothalamic suprachiasmatic
nucleus (SCN) in mammals. The endogenous period generated by the
pacemaker is close to, but generally not equal to 24 h and the
biological clock therefore needs to be daily adjusted (entrained)
by external cues. The daily alteration of light and darkness due to
the rotation of our planet on its own axis in relation to the sun
is the most prominent "zeitgeber" which adjusts the phase of the
circadian rhythms to the astronomical day length, a process known
as photoentrainment. In mammals, light is perceived only through
photoreceptors located in the retina. Light information is mediated
to the SCN via the retinohypothalamic tract (RHT) by activation of
the classical photoreceptor system of rods and cones and a more
recently identified system of intrinsic photosensitive retinal
ganglion cells (ipRGCs) using melanopsin as a photopigment.
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