The Big Picture: Pausing a Movie
Sleep is like pressing pause on a movie. The picture freezes, the player almost stops using power, but your place is kept. Press play and you are back in a second.
Sleep is the everyday name for the S3 state from Section 2 (on PCs that support it). Everything you were doing stays in RAM, and RAM is kept alive with a tiny trickle of power.
3.1 What Gets Preserved (RAM Stays Powered)
What it is: Before the PC drops into sleep, Windows must make sure nothing is lost. Here is what is kept and where.
| Thing | Where it is kept during sleep |
|---|---|
| Your open apps, windows, unsaved documents | In RAM |
| The Windows kernel, drivers and their data | In RAM |
| CPU registers and the position of the running code | Saved into RAM by Windows, then the CPU is powered off |
| Device settings (what mode the GPU or NIC was in) | Each driver saves them into RAM |
| Time | The real-time clock keeps going on the motherboard's CMOS battery (Chapter 4) |
How RAM survives: DRAM (Chapter 3) stores bits as tiny electric charges that leak away, so it must be refreshed constantly. Normally the memory controller does that. When the CPU is off, the RAM chips switch to self-refresh: they refresh themselves using very little power, taken from the standby supply (the purple +5 V standby wire in Chapter 4, Section 5.1).
The big risk: if the power is cut during S3 (a laptop battery dies, or the plug is pulled from a desktop), RAM goes blank and everything unsaved is lost. Windows reduces this risk on laptops with a combined setting called hybrid sleep (see Section 4.5 for the idea), and by switching to hibernate when the battery gets very low.
3.2 What Happens When You Go to Sleep (Step by Step)
1. Trigger Lid closed / Start -> Sleep / idle timer fires / sleep key
2. Power Manager Decides: go to S3
3. Notify Apps and services are told "sleep is coming"
(Windows APIs let programs react; services may be paused)
4. Drivers Each driver gets a power IRP (IRP_MJ_POWER, Chapter 13)
and saves its device state
5. Devices Devices go to D3, the "off" device state (Section 8)
6. CPU The other cores are stopped; CPU state is saved
7. ACPI acpi.sys runs the firmware's sleep method; the chipset
writes the sleep command
8. Hardware CPU off, RAM in self-refresh, standby power only
The reverse happens on wake:
1. Wake event power button / key / lid / timer / network (Section 10)
2. Firmware Starts the CPU at a special wake address (short, not a full boot)
3. Windows Restores CPU state, then wakes each driver
4. Drivers Restore device settings, devices return to D0 (full on)
5. Apps Notified "we are back"; Wi-Fi reconnects; screen lights up
The firmware does not run a full startup here. It only does the minimum, which is why wake is so much faster than boot (Chapter 5 and Chapter 6 describe the full boot that sleep avoids).
3.3 Wake Latency (Fast Resume)
What it is: Wake latency is the time between "I pressed a key" and "I can work". For S3 it is usually about one to three seconds, but it can be longer if a driver is slow.
| Why it is fast | Why it can still be slow |
|---|---|
| RAM already holds everything; nothing is read from disk | A slow driver or device (for example, a USB device or network adapter) takes time to wake |
| No firmware self-test and no loader | The screen or GPU takes time to restart |
| Services were only paused | An app does heavy work right after waking |
Waking from hibernate (Section 4) is slower because RAM must be read back from the drive. Modern Standby (Section 5) wakes almost instantly because the PC never fully slept.
3.4 What Wakes a Sleeping PC
A short preview (Section 10 has the full list and the commands):
| Wake source | Example |
|---|---|
| Power button | Always works |
| Keyboard or mouse | Press a key, move the mouse (if allowed) |
| Lid | Open the laptop lid |
| Timer | A scheduled task or Windows maintenance |
| Network | A special "magic packet" (Wake-on-LAN) |
3.5 Sleep vs Modern Standby vs Hibernate (At a Glance)
| Sleep (S3) | Hibernate (S4) | Modern Standby | |
|---|---|---|---|
| State | S3 | S4 | S0 Low Power Idle |
| Battery use while "off" | Small | None | Higher than S3 |
| Survives power loss | No | Yes | No |
| Wake speed | 1-3 s | Slower | Instant |
| Runs background work | No | No | Yes |
Check your own machine: run powercfg /a. If it lists "Standby (S3)", the sleep you use is classic S3. If it lists "Standby (S0 Low Power Idle)", your "sleep" is Modern Standby (Section 5).
3.6 Why Won't My PC Sleep? (powercfg /requests)
What it is: Programs and drivers can ask Windows "please do not sleep yet" (a power request, for example while a video plays or a download runs). The command shows who is asking. It only reads.
powercfg /requests
On some PCs this command needs an administrator window. It lists groups such as DISPLAY, SYSTEM, AWAYMODE, EXECUTION, PERFBOOST and ACTIVELOCKSCREEN, each with the program or driver holding a request, or "None."
| If you see | It means |
|---|---|
SYSTEM: with a program or driver name | That item is keeping the PC awake |
DISPLAY: with a player or browser | The screen will not turn off (a video is playing) |
None. everywhere | Nothing is blocking sleep |
Summary
| Term | Plain meaning |
|---|---|
| Sleep (S3) | RAM kept in self-refresh; everything else off |
| Self-refresh | RAM refreshes itself at very low power |
| Wake latency | Time from wake event to usable PC (about 1-3 s for S3) |
| Power IRP | The message that tells each driver to save and power down |
| D3 | The "off" state a device enters when the PC sleeps |
powercfg /requests | Shows who is stopping the PC from sleeping |