Harms of too dim brightness: If the brightness of your gadget is too low, you will end up straining your eyes. This happens because you may have to focus a little strainfully. Although, this will not cause any long term damage but can lead to headaches.
Should you keep it bright? Again, this is not ideal either. If you keep the screen brightness of your gadget at higher than 50 per cent, you still may not risk a permanent damage but many short term problems, such as digital eye strain, eye irritation and dry and fuzzy eyes.
Ideal screen brightness: Experts believe that the ideal screen brightness of your gadget depends on your ambient light. As a simple rule, remember that the brightness of your gadget should be equivalent to the light surrounding you, such that both the lights' radiance is similar. By doing so, they will blend in, causing the least damage to your eyes.