Bright Light Induction of Strong (Type 0) Resetting of the Human Circadian Pacemaker

TLDR

The authors measured the human circadian pacemaker’s response to light in 45 resetting trials, each consisting of an initial phase assessment, a three‑cycle bright‑light stimulus (5 h per cycle), and a final phase assessment. The bright‑light stimulus induced strong type‑0 resetting, with phase‑shift magnitude and direction highly dependent on initial circadian phase and modulated by ordinary room light, revealing that the pacemaker’s sensitivity to light is far greater than previously recognized and suggesting important therapeutic implications.

Abstract

The response of the human circadian pacemaker to light was measured in 45 resetting trials. Each trial consisted of an initial endogenous circadian phase assessment, a three-cycle stimulus which included 5 hours of bright light per cycle, and a final phase assessment. The stimulus induced strong (type 0) resetting, with responses highly dependent on the initial circadian phase of light exposure. The magnitude and direction of the phase shifts were modulated by the timing of exposure to ordinary room light, previously thought to be undetectable by the human pacemaker. The data indicate that the sensitivity of the human circadian pacemaker to light is far greater than previously recognized and have important implications for the therapeutic use of light in the management of disorders of circadian regulation.