When we first started racing widowhood it was customary to darken the loft partially when the birds were inside. Many lofts still do it, but we no longer do. We feel the sun does more good and we find that if we pull the blinds down only when the other birds are outside so the widowers will not be excited that this is sufficient. We are, however, pulling the blinds down in the evening. This is after they have exercised and are fed and had sufficient opportunity for drinking. This darkens the loft.
Taken from Widowhood flying
by Mark Gordon