My 16 week old will not go off to sleep easily in the day. I've tried everything. Some things work for a while and then wear off. I've tried different sleeping positions, just holding him until he's calm and then putting him down, darkened rooms, sleeping in cot, lullaby cot toys, letting him cry but he just refuses to sleep in the day without a fight. At night I can put him in his cot and turn out the light and he goes to sleep. Any tips would be grately appreciated.