The knees have it

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Science  16 Jan 1998:
Vol. 279, Issue 5349, pp. 297
DOI: 10.1126/science.279.5349.297m

The circadian clock of humans is approximately a 24-hour period, but is kept synchronized with the solar days by daily entrainment to our natural light dark cycle. It has always been supposed that this entrainment occurs through light stimulation of the retina, but Campbell et al. (p. 396; see the commentary by Oren, p. 333) now demonstrate the surprising finding that application of light to the back of the knees only can entrain endogenous circadian rhythms in temperature and melatonin.

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