Windows Sleep "Z" letters

When you select “Sleep” in your Windows PC’s power options, what happens, exactly? We’ll explore what it means—and whether your PC has pleasant dreams.

Sleep Is a Low-Power Mode

Sleep mode is different from shutting down your computer, which powers off the computer completely. When you shut down your computer, it takes longer to start back up, and you have to close all your programs and files first.

In contrast, sleep mode is a power-saving state that allows a computer to quickly resume full-power operation (usually within several seconds) when you want to start working again. When you enter sleep mode in Windows, your PC does the following:

  • The display turns off.
  • The PC saves its current state to memory.
  • The PC stops sending power to most of the hardware devices.
  • The PC goes into a low-power state.

As long as some power continues to flow to your PC (for example, your desktop PC is still plugged into the wall or your laptop still has battery life), you won’t lose any work or the state of your machine. When you wake your computer back up, you can pick up right where you left off.

RELATED: How to Turn Off a Windows 11 PC

The Magic of ACPI Power Modes

When you use sleep mode in Windows, it’s as easy as selecting an on-screen option or pushing a button. But have you ever wondered how sleep mode works on a lower level?

Sleep mode is governed by ACPI Sleep States. ACPI (Advanced Configuration and Power Interface) is a power management specification that operating systems use to control the power states of hardware. There are six primary ACPI power states in the standard, which are often referred to by their abbreviations. They are:

  • S0: Working
  • S1: Sleep
  • S2: Sleep
  • S3: Sleep
  • S4: Hibernate
  • S5: Soft Off

S0 is the power state your PC is in when it’s turned on and running normally. S1, S2, and S3 are progressively lower power states. S1 and S2 aren’t used very often. In S1, your PC’s CPU stops executing instructions, but it doesn’t lose power. In S2, your PC’s CPU also loses power. In S3 (often called Standby or Suspend to RAM), most of your PC’s components lose power except for the memory, which is kept refreshed with a minimal amount of power. S3 is the most commonly used mode (determined by the hardware manufacturer of your PC) when you put your PC to sleep.

S4 is the hibernate state. In this state, your PC saves the contents of its memory to a hibernation file on the hard drive and then powers off. When you wake your PC up from hibernation, it reads the hibernation file and restores your open programs and files from the hard drive. S5 is the “soft off” state. In this state, your PC is completely powered off. It’s the same as if you had pressed the power button to turn off your PC.

RELATED: You're Shutting Down Wrong: How to Really Shut Down Windows

How to Put Your PC to Sleep

Now that you know what happens when you put your PC to sleep, how do you actually do it? Luckily, it’s really easy on both Windows 10 and Windows 11. By default on many machines, you can simply press the power button one time and your PC will automatically go into a sleep mode. If that doesn’t work, open the Start menu, click the power icon, then select “Sleep.” Or you can press Windows+X on your keyboard, then press “U,” then “S.” Your PC will go to sleep instantly.

Pleasant dreams!

RELATED: How to Put a Windows 11 PC to Sleep

Profile Photo for Benj Edwards Benj Edwards
Benj Edwards is a former Associate Editor for How-To Geek. Now, he is an AI and Machine Learning Reporter for Ars Technica. For over 15 years, he has written about technology and tech history for sites such as The Atlantic, Fast Company, PCMag, PCWorld, Macworld, Ars Technica, and Wired. In 2005, he created Vintage Computing and Gaming, a blog devoted to tech history. He also created The Culture of Tech podcast and regularly contributes to the Retronauts retrogaming podcast.
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