This subject is touched all the time, so if I type too fast or give u lingos u don't understand please use the SEARCH button.
S1 = light sleep, not worthwhile as a goal if u want machine to sleep.
S3 = best sleep mode. Relatively quick wake up and save most power.
When machine is sleep it can be WOKEN up by several methods.
1. Of course the OS itself can be told to wake up the machine to perform tasks such as recording. This is built-in, usually there is nothing for you to do. You tell your recorder to start the recording at certain time, it would do so, sometimes you have to check a box that says "Wake up machine to perform this task."
2. A mouse/keyboard/IR sensor hooked up to USB. USB port must be configured to be powered at all times.
3. From outside your LAN, u should configure your NIC to wakup when there is network activity. This is not perfect however as there maybe some false trigger but depends on your expertise in configuring it, it maybe minimized to be good enough.
BTW, at least 1/2 of the people here leave their machines on 24x7 as configure to sleep can be troublesome. Your mileage will vary.