Circadian rhythm
Circadian rhythm disruption is primarily caused by the wrong timing of light in reference to the circadian phase. It can also be affected by too much light, too little light, or incorrect spectral composition of light. This effect is driven by stimulus (or lack of stimulus) to photosensitive ganglion cells in the retina. The "time of day", the circadian phase, is signalled to the pineal gland, the body’s photometer, by the suprachiasmatic nucleus. Bright light in the evening or in the early morning shifts the phase of the production of melatonin (see phase response curve). An out-of-sync melatonin rhythm can worsen cardiac arrhythmias and increase oxidized lipids in the ischemic heart. Melatonin also reduces superoxide production and myeloperoxide (an enzyme in neutrophils which produces hypochlorous acid) during ischemia-reperfusion.