“Our study shows that a two-hour exposure to light from self-luminous electronic displays can suppress melatonin by about 22 per cent,” Figueiro says in a statement. “Stimulating the human circadian system to this level may affect sleep in those using the devices prior to bedtime.” ... "Figueiro recommends dimming your backlit devices at night to minimise melatonin suppression"
W. Chen, J. Chu, J. Luan, H. Bai, Y. Wang, and E. Chang. Proceedings of the 18th international conference on World wide web, page 681--690. New York, NY, USA, ACM, (2009)