Hey Ladies,
My GF is going through the same thing with her 5.5 month old. I read somewhere that around 6 months they develop "night" sleep cycles. They get day sleep cycles at around 6 weeks, which is one of the reasons that a normally sleepy baby starts only having 20 minute sleeps.

Apparently the same thing happens at around 6 months where a normally sleepy baby sleeps through the night, they start waking up after a sleep cycle or too.

I don't have much advice, other than the same rules I applied to get Caden to sleep during the day - dont pick them up but settle them in the cot. Put them straight back in to bed even if they are wide awake (clear night time messages). The hardest thing - persevering with the crying. Even if you are going in and out of their room every 4 minutes or so to settle them, it is better than feeding them off to sleep or some other method - cos that way they learn to settle themselves to sleep rather than needing you to do it for them.

There are some great services that offer support over the phone - Ngala here in WA or Tresellian. They will have some tips on what is appropriate.

Good luck!