Screens may look nicer when their brightness is up, but the high intensity of light is harmful for your eyes. 1)
Especially high frequency blue/violet light that carries more energy per photon. 2). Human eyes are designed to block UV light from reaching the retina. Thus, the danger of creating reactive oxygen species (ROS) on the retina is highest from the blue light frequency range. 3)
In order to reduce retinal nerve damage, the goal is to let in as little light into your eyes as possible, where you can still see the detail you need to see.
Eyes evolved to have a relatively even intensity of light from the entire visual field, as obtained from sunlight's dispersion through the atmosphere.
While working at your monitor, light comes in through both ambient and monitor sources. The summation of the light from the entire visual field contribute to how much your pupils contract. The retinal nerve cells responsible for pupillary-light-reflex are the Photosensitive Ganglion Cells.
If the light distribution reaching the retina is uneven, some cells may receive an unhealthy amount of radiation which can lead to cell death. In the case of electronic displays, the distribution of retinal light intensity can be uneven if the ambient light and screen brightness are not balanced.
For example, if you are watching a bright TV in a dark room. As an extreme example, staring at the sun. The opposite would be too dim of a display with intense ambient light, where peripheral nerves of the retina may be stressed.
“As in a photographic lens, visual acuity is affected by the size of the pupil. Optical aberrations of the eye that decrease visual acuity are at a maximum when the pupil is largest (about 8 mm), which occurs in low-light conditions. When the pupil is small (1–2 mm), image sharpness may be limited by diffraction of light by the pupil (see diffraction limit). Between these extremes is the pupil diameter that is generally best for visual acuity in normal, healthy eyes; this tends to be around 3 or 4 mm.” 5)
The best situation is where the monitor light and the ambient light have matching intensity. But it's still not optimal if both sources are extremely bright. A low ambient light and a matching low screen brightness, which still allows you to see the detail you need to see, is the best option for eye healh.
In practice, with most computer monitors and most indoor lighting, you will be dialing the brightness level to zero. Monitors are just too bright, unless sunlight is shining through your window in the background. You can also play with the contrast to optimize image clarity and readability.
If you practice this from an early age, you will find that most people in the era-of-screens have limited vision in low light. Manufacturers are starting to take note, with now common features such as screens with automatic brightness, and a blue-light cycle that follows the circadian rhythm.
The physiology of eye strain, or asthenopia, is not fully understood. 6)
Asthenopia has been categorized into eye surface and internal symptoms. 7) 8) Explanations for internal eye strain include muscular ache from accomodative, convergent and pupillary contractions. External eye strain consists of “sensations of dryness and irritation on the front of the eye and caused by compromised conditions in the viewing environment.” 9)