Almost everywhere you look there are sources of blue light. In cell phones, tablets and computers, blue light is emitted. Blue light is on the higher intensity end of the EM spectrum of light. The white light from the sun is a spectrum of light that blends with blue light included. The problem with cell phones and other devices is that the light they emit has much more highly concentrated blue waves than sunlight. Blue light doesn't get absorbed by the cornea of the eye; it actually passes through to the retina causing cell damage over time. When cell phones or other devices are used late at night, sleep disruption and other health problems can result.
Blue light is how our brains know that it's day or night. When it's night, there is no blue light from the sun, and our bodies produce melatonin which regulates sleep and our circadian rhythm. In addition to possible eye damage, using cell phones and other screen devices at night disrupts the healthy balance of melatonin and interferes with sleep. Sleep deprivation, over an extended period, has been linked to heart disease, diabetes and even some forms of cancer.