White light contains blue in its spectrum.
How much blue will depend on the type of white it is, usually expressed as a colour temperature, but also can be given a name such as daylight. Screen backlighting that appears nice and bright typically has quite a lot of blue in it.
The concern is that the eye receiving stimulus of blue light causes production of Melatonin that impacts on circadian rhythms, and it's believed that doing this at night is likely to cause sleeplessness. The thinking is that holding a small device that probably has a bright screen to allow viewing outdoors close to your eyes to read emails could deliver a fair dose of blue light.