Reading while lying down is not considered a proven cause of permanent vision deterioration. There is no convincing evidence that the habit of reading in bed in healthy adults "damages vision," damages the retina, or directly causes myopia.
Yes, reading in dim light is possible; however, there is no convincing evidence that this in itself damages the eyes or permanently impairs vision in adults. The National Eye Institute clearly states that reading in dim light does not harm the eyes, although it may tire them more quickly.
It's impossible to say one form factor is inherently safer for hearing than another. The main risk is determined not by whether the speaker is located inside or around the ear, but by the actual volume at the eardrum and the duration of listening.
For an adult, a reasonable guideline is considered to be an average level of up to 80 decibels with a total listening time of no more than 40 hours per week.
For most people, regular vitamin C intake does not prevent the common cold. A major Cochrane review found no clinically significant reduction in the risk of developing the disease in the general population: among more than 10,000 participants, the risk of catching a cold in those who regularly took vitamin C was virtually the same as in the control group.
For a healthy adult, there's no single best sleeping position: both side and back sleeping can be normal. The choice varies depending on the specific problem. For snoring and especially positional obstructive sleep apnea, the side is usually preferable.
