Turn off your monitor and necessarily kill any process that isn't needed. There's not much more you can do.
However, you need to take into consideration the following:
Computer Power Consumption is rated in the Watts/hour level. Let's assume that your Power Supply unit can go up to 600 Watts and that for some unfathomed reason you would be using up all that power during the night (you won't even be using half of it):
- To calculate the kWh: (Watts x Used
Hours) / 1000
So, if you leave it running for 8 hours during the night: (600 * 8) / 1000 = ~5 kWh
If you pay $0.20 USD per kiloWatt, that night cost you 1 dollar.
And because I went to the extreme and assumed full power consumption, you can see how this is pretty much irrelevant. Any minor changes you do will not have a big impact on the cost over such a small period of time. If you add them over an year, well then... that's where the differences start to be more meaningful.
In any case let's do a more approximate calculation. With monitor turned off, speakers turned off, and just the machine working in the background along with your modem (which usually consumes around 15 watts. See note at the end), I estimate you will be using somewhere around 270 watts.
(270 * 8) / 100 = 1.656 kWh
Multiply that for your local kWh cost. If it's $0.20 USD, the night will cost you ~33 cents :)
Despite all this, I share with you a certain... how should I put it?... irrational feeling towards savings. So, one thing I do is keep a window partially opened and detached the side panel from my computer. This increases cooling and makes the fans do less work, which results only in marginal power savings (because fans don't really use that much). But hey!
Note: To find power consumption of a device without a watt or kilowatt indicator, multiply Amps by Volts. That will give you watts/hour.