The human circadian system adapts to prior photic history

Non‐technical summary The human biological clock organizes and regulates the timing of many biochemical and physiological processes, including the timing of sleep, on a daily basis. Light is the strongest time cue to the circadian clock that keeps these rhythms entrained to the 24 h day. Light exposure at night results in ‘resetting’ of the clock (phase shifting). In the current study, we examined the effects of exposing subjects to two different light levels (very dim light vs. typical room light) before exposure to a 6.5 h light exposure at night. Results showed that the very dim light level, compared to the typical room light level, prior to the light exposure at night caused a substantially greater phase shift of the melatonin rhythm and substantially greater acute melatonin suppression. Thus, prior dim light history sensitizes the human biological clock to the effect of a subsequent light exposure. Light is the most potent stimulus for synchronizing the endogenous circadian timing system to the 24 h day. The timing, intensity, duration, pattern and wavelength of light are known to modulate photic resetting of the circadian system and acute suppression of melatonin secretion. The effect of prior photic history on these processes, however, is not well understood. Although previous studies have shown that light history affects the suppression of melatonin in response to a subsequent light exposure, here we show for the first time that a very dim light history, as opposed to a typical indoor room illuminance, amplifies the phase‐shifting response to a subsequent sub‐saturating light stimulus by 60–70%. This greater efficacy provides evidence for dynamic adaptive changes in the sensitivity of circadian ocular photoreception. This plasticity has important implications for the optimization of light therapy for the treatment of circadian rhythm sleep disorders.