Any 11hr period should be about the same to Lily if she's given the time to adjust gradually, since at five months old she's certainly not telling time yet. We tried simply pushing Lily's bed time later, but she kept waking up at about six. We have good thick curtains on the windows but still enough light gets through that it stops feeling like night by ~6am. Perhaps she's just going to wake up with the sun?

So I put tinfoil on all the window panes:

Now our room is as dark in the morning as it is at night. We can push Lily's bedtime back gradually so she and Julia can see more of each other, and I can sleep later in the morning. We're only a few days in but it seems to be working.

(You might worry that it would get stuffy with the windows closed, being summer in Boston. We're leaving the windows open with a fan until the last feeding in the early morning. As it gets colder and we want the windows closed anyway this will be less of an issue. And bonus: closing the windows after bringing in cool early-morning air and then leaving the windows closed with reflective foil on them through the hottest part of the day keeps our south-facing room nice and cool all day for naps.)

Update 2016-09-29: here's how to do it with velcro instead, so you can have light during the day.