Many thing could happen if you leave your computers overnight. Apart from paying more on you electrical bills. you computer's life will also lessen if you don't let them to cool down and keeping them on for long periods on time.If you left them open at night, this time give opportunity for viruses and hacker to exploit your unattended machine especially if its online even you firewall is enabled.
There are lots of reason why you shouldn't do them, learn more on the message board, link below. You could also share you ideas there too.
Link: Risk of Leaving Computer On Overnight; Internet Securit
