Problem with Messenger signing me out.

For some reason my messenger account keeps signing me out when it is inactive for a period of time....it's a pain because I cannot hear if someone tries to send me an instant message and my screen saver is on...I have done nothing to any settings on my computer. This has just been happening recently. Is there someplace I can look to change some setting to leave my messenger on until I decide to turn it off myself. I have never had this problem before and I am baffled by it. Can someone help? Would be much appreciated. Thanks