What happens when a computer goes to sleep?

Question

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What is the difference between (these four) sleep states?

What exactly (hardware and software-wise) happens when a computer goes to sleep? Do the actual processes turn off, just the screen?...What exactly happens?

Answer 1

Sleep mode can go by many different names, including Stand By (for Microsoft Windows 98-Server 2003), Sleep (for Mac OS 8-Mac OS X, Windows Vista, Windows 7, Windows Server 2008), and Suspend (Windows 95, Linux). When placed in this sleep mode, aside from the RAM, which is required to restore the machine's state, the computer attempts to cut power to all unneeded parts of the machine. Because of the large power savings, most laptops automatically enter this mode when the computer is running on batteries and the lid is closed.

A computer must consume some power while sleeping to run the circuits required to recognise and act on a wake-up event; a sleeping computer is a case of a device consuming standby power.

Sleep is a power-saving state that allows a computer to quickly resume full-power operation (typically within several seconds) when you want to start working again. Putting your computer into the sleep state is like pausing a DVD player—the computer immediately stops what it’s doing and is ready to start again when you want to resume working.
– From the online Windows 7 documentation.

Answer 2

When a computer goes to sleep, all the processes are suspended, and the state of the OS and all the processes are stored in your PC's Random Access Memory (RAM). Since RAM is very fast to access, a computer can resume from sleep very quickly. However, RAM requires constant power to maintain its data, so a computer in sleep still uses some power, but not as much as when it is running. Obviously, since the OS is suspended, everything else is suspended too, such as the screen, peripherals and most input devices (Windows controls which devices can still operate when in sleep).
For comparison, hibernation does a similar thing, but stores all this data in the Hard Drive, which takes longer to read all the data in case of a resume, but does not require any power to maintain, so the computer is effectively off when hibernated.