The purpose of this study was to investigate the effect of bright artificial light exposure on the rhythms of 6-sulphatoxy melatonin cortisol excretion in urine. Six healthy males were exposed to light (> 3,000 lux) from 1900 to 0200 h (sunset 1928 h) on one occasion. The artificial light delayed the onset of 6-sulphatoxy melatonin excretion. On the next evening the onset of 6-sulphatoxy melatonin excretion in normal light/darkness was delayed by 1 h. The timing of the peak excretion of cortisol was not affected by the light treatment; however, cortisol excretion rate was maintained at a signficantly higher rate in the morning afternoon after the treatment. These results demonstrate the inhibitory action of high intensity light in humans suggest that one 6-h period of extra light in the evening can phase delay the melatonin onset.