do you have a night time routine??
until 3 weeks DD2 settled whenever but one night at 3 weeks she woke in time to have a bath when DD1 did and so started our night routine, from that night on she went down just after DD1 did, around 6.45pm, she knows this means sleep time now, maybe try a wind down routine an slowly bring the time back earlier each night sleep deprivation is awful, HTH