 We will test the hypotheses that: 1 light-induced activation of the neurobiological sleep-wake regulatory system, as indicated by increased alertness, faster reaction time, suppression of EEG alpha activity, microsleeps and slow rolling eye movements, and suppression of pineal melatonin, is most sensitive to retinal exposure to short wavelength blue light 460 nm compared to equal photons of other colors of visible light; 2 light-induced activation of autonomic and hypothalamic-pituitary-adrenal axis measures of arousal, as indicated by increased heart rate variability, core body temperature, blood pressure, respiration rate, plasma cortisol levels and urinary catecholamines, is most sensitive to exposure to short wavelength blue light 460 nm compared to equal photons of other colors; 3 phase shifts of the human circadian pacemaker, as assessed by changes in temperature, melatonin and cortisol rhythms, are most sensitive to exposure to short wavelength blue light 460 nm compared to equal photons of other colors. The resultant action spectra will help to identify the photoreceptor mechanisms by which light activates arousal and circadian resetting, these non-image-forming physiological responses and enable us to distinguish between major candidate photoreceptive mechanisms, including potential novel photoreceptor systems, that might mediate such responses. 