My 6 week old just wont sleep at night!

Help, my 6 week old has started being awake from 5pm till 1am in the morning, and the only reason she goes to sleep then is because i give in and put her in the car or let her scream herself to sleep.....horrible i know sad but i'm just so tired. Then she only sleeps for two hours and then is up again for a an hour or two...sometimes 3!

Im losing it, its not like she sleeps all day either. Just two hours here and there, i'm one tired mummy....please help.

Thanks Happyhead im going to take her to see the plunket nurse today, and i will give that 6.30pm feed a try. She wont sleep after a feed or be rocked to sleep that late at night. Could she get over tired maybe?