This is some valuable information for users of iPad or Samsung Galaxy SII.

"Our study shows that a two-hour exposure to light from self-luminous electronic displays can suppress melatonin by about 22 per cent. Stimulating the human circadian system to this level may affect sleep in those using the devices prior to bedtime," researchers said.
This problem also has an easy solution which is dimming the backlit devices at night to reduce melatonin suppression.
Source-Medindia