According to the researchers, from Manchester University, this blue light from devices is not the main problem.
They exposed mice to different light settings to study the impact.
The controversial findings, in Current Biology journal, suggest the answer lies in how warm and bright a light is.
Why do the study?
Everyone has a natural, daily wake-sleep cycle.
Their body clock synchronises with the environment so they are alert in the daytime and sleepy at night.
But experts have long suspected exposure to artificial light might be upsetting this alignment.
And there is a popular view artificial blue light – the type that comes from computer screens and mobile phones – has a particularly strong effect.