In the category of “this problem has been around for years but no-one has noticed it”, I discovered very recently that IE raises an availability request when it downloads a file. This is the desired behavior – you don’t want the PC to automatically sleep during the middle of a download. However, once the download is finished, the availability request isn’t dropped until after you dismiss the ‘download completed’ dialog box. So for large downloads that you start before you go to bed, be sure to check the box that automatically closes the download dialog box so your PC can enjoy a good night’s rest when it’s done downloading. A bug has been filed against IE 9.