Hey Toby, welcome to PCHF.
All temperatures are normal Toby? You can use speedfan to check all your temperatures at once:
Download SpeedFan - Access temperature sensor in your computer
Also, take a look at Task Manager, by pressing [CTRL]+[ALT]+[DEL]. Click on the
Performance tab when your PC is supposed to be idle, and check your
CPU usage and memory to make sure that nothing is using these.
You mentioned downloading overnight. Please note that if your using software such as Torrent's and illegal file sharing, then I'm afraid we won't be able to support you on this one unless you uninstall such software.
EDIT: Marked as Answered.