Automatic Download without password?

I have limited internet usage. I get 10 GB a month during peak times, with unlimited usage between 2 AM - 8 AM.

Anyway, I used to have iTunes set up to automatically open at 2 AM, check for and download my purchases. This worked great, until recently. A couple of months ago, iTunes started asking me for my password everytime I opened it up and it checked for purchases. No matter what I do, I can't find a way to get around this. This means that if I want to download my purchases without using all of my bandwidth, I have to stay up until 2 AM, which is not typically an option.

Does anyone know a way I can set iTunes (10.5.2) to check for and download my purchases WITHOUT having my manually put in my password every time?

Thanks!