Sunglasses are a novelty only recently introduced in human history. They deprive your circadian rhythm of meaningful data from light (e.g., "it's time to be awake"). And the info our bodies receive via sunglasses contradicts the info we receive from sunlight shining directly on our skin, leading to a host of possible outcomes (e.g., sunburn, cancer).

Physically speaking the sun is our planet's source of life. So we should respect it and as with other interventions we should exercise the precautionary principle with sunglasses.