Effects of popliteal illumination on cerebral near-infrared spectroscopy

The endogenous clock of humans can be influenced by light application to the popliteal region. Previously, it had been assumed that light must fall directly into the eye in order to influence the inner circadian clock. This study, published in 1998, has not been reproduced. We investigated the effects of laser light illumination (785 nm for 5 min) on cerebral near infrared spectroscopy (NIRS) in 33 volunteers. There were significant (p < 0.001) delayed (2 min) alterations in NIRS parameters during popliteal light illumination. This could be further evidence for extraocular phototransduction in humans.