There’s no need to go mucking about in your power settings to temporarily keep your machine awake, let Coffee perk it up and keep everything active until your downloads are finished.

Practically everyone has left their machine on to finish a download, just accepting that wasting the electricity/money to run it all night is better than sitting up until 3AM when the files are done downloading. Coffee offers a simple solution in the form of a temporary sleep/hibernation blocker for
Windows PCs. Run it, point it at the network adapter that data is flowing through, and give it a minimum data threshold before it shuts off and allows the computer to sleep or hibernate on its own.

Visit the link below to grab a copy. Coffee is a Windows only application and required Microsoft .NET.
Coffee [SourceForge via Addictive Tips]