i do think the darkness can bring ybs on as most do not let any air in to the loft.
say they was 40 birds in loft and the amount off air was ok then you close the loft up and now the birds will be stressed as the will be not much air coming in any more.
i do not like curtains as they stop air coming in and out but i do like Venetian blinds as air can still pass if you do not close them up all the way