Help with Mac Mini

I recently purchased a Mac Mini to put out in my living room to hook up to my TV. I've noticed that when I put the Mini to sleep and switch sources to watch television that the Mini will wake up on it's own for about ten seconds and then go back to sleep. I have a 1 TB Western Digital external hard drive hooked up to it and thought maybe that was causing it to do that, but I'm not sure as I'm relatively new to Mac computers. Anyone have any advice or answers as to why this is happening? Thanks.