Scientific studies have shown that light has a biological effect on our body, among other things helping to regulate our hormone balance. The hormone melatonin plays a significant role in regulating our sleep/wake cycle, and the light energy that we require for this process is largely absorbed through our eyes. Another key factor in this process is a light-absorbing pigment in the eye called melanopsin, which has been shown to be most active in the short-wavelength portion of the visible spectrum. That means that the blue light which reaches our retina also plays a part in ensuring our psychological well-being, which is why light therapy is successfully employed to treat winter depression and insomnia.
UV light is also involved in the production of vitamins, which means that light stimulation also has an important effect on our metabolism. In summary, our bodies need blue light.
To take one concrete example, many people are familiar with the concept of age-related insomnia, which refers to the fact that elderly people seem to require less sleep. The light that reaches the human eye seems to play an important role in this phenomenon. The cataracts that older people typically suffer from, prevent some of the blue light from reaching the retina – and the intraocular lenses used to treat patients with cataracts also have the effect of blocking blue light. This might causes perturbations in the day-night rhythm of these mostly elderly individuals and alters the amount of sleep they need.
But as in so many other cases, the rule that applies here is "everything in moderation."