This damp still air is ideal for the growth of bacteria and fungi and as a consequence airborne pathogens will increase. This environment will compromise the health of the pigeons and they won't perform at their best. Another drawback to limiting the amount of natural sunlight in the loft, is that the it reduces the amount of ultraviolet rays and these are a great antibacterial. The ultraviolet quickly destroys any bacteria that are lying dormant in the loft or carried in the air. Furthermore, lofts that are kept darkened in the morning, miss the early sun, which usually warms and dries out the loft.
Pathogens therefore have an ideal environment in which to breed.
It is interesting to speculate that the rise in youngbird sickness appears to correlate with a rise in the use of the darkness system for youngbirds. Another function that the sun provides for youngbirds, is to stimulate the production of vitamins, and growth hormone. In a normal youngbird loft you will see youngbirds taking advantage of the suns rays. They will often be seen sitting in the sun with a wing outstretched to catch the most sun they can. Lofts that are darkened during early morning, or during part of the day, miss a large proportion of this important stimulus.