# Sleep, hibernate and Fast Startup: three switches people confuse

> What each power state does, what Fast Startup actually writes to disk, and why it breaks dual boot and update troubleshooting.

**Tested on**

- Build: 26300.9457 / 26100.33650
- Edition: Pro (26H2) / Server
- Architecture: x64
- Evidence: E1
- Verified: 2026-10-29

## The short answer

Three different things:

| Feature | State | RAM kept powered | Writes to disk |
|---|---|---|---|
| **Sleep (S3)** | suspend to RAM | yes | no |
| **Hibernate (S4)** | suspend to disk | no | hiberfil.sys |
| **Fast Startup** | hybrid shutdown | no | partial hiberfil.sys |

Sleep is instant and fragile (battery drain). Hibernate is slow to resume and survives power loss. **Fast Startup is not sleep at all** — it is a shutdown that keeps the kernel session on disk.

## Check what the machine supports

```powershell
powercfg /a
```

Typical output on a Modern Standby laptop:

```
待机 (S0 低电量待机)
休眠
快速启动
```

On a desktop:

```
待机 (S3)
休眠
快速启动
```

If `休眠` is missing, hiberfil.sys is disabled (`powercfg /hibernate off`) or the disk is too small.

## Fast Startup: what it really does

On shutdown with Fast Startup enabled:

1. User sessions close (that part is a real logoff).
2. The **kernel session** is hibernated to `hiberfil.sys`.
3. Next "boot" resumes that image instead of initialising drivers from scratch.

Consequences people hit:

- **Updates** that need a full restart may not fully apply until a real restart (`shutdown /s /full` or Restart).
- **Dual boot**: the hibernated image keeps NTFS mounted; Linux may refuse to mount or mount read-only.
- **Driver changes** may not take effect on "shutdown then start".
- **Troubleshooting** a boot problem is harder because the boot is not a boot.

```powershell
# is Fast Startup on? (1 = enabled)
Get-ItemProperty 'HKLM:\SYSTEM\CurrentControlSet\Control\Session Manager\Power' -Name HiberbootEnabled
# a real, full shutdown
shutdown /s /full /t 0
```

## Hibernate: the honest trade

Hibernate writes RAM content to disk and powers off. Resume reads it back.

- **Pro**: survives power loss, zero battery drain, resumes your session.
- **Con**: resume time proportional to RAM and disk speed; `hiberfil.sys` is roughly 40% of RAM by default.

```powershell
powercfg /hibernate on
powercfg /hibernate size 60     # percent of RAM
powercfg /hibernate off         # deletes the file, disables fast startup too
```

The file sits at `C:\hiberfil.sys` and is not visible in Explorer without showing protected OS files.

## Which to use

| Machine | Recommendation |
|---|---|
| Desktop, always plugged in | Sleep on, hibernate optional, Fast Startup off if you dual boot |
| Laptop, Modern Standby | Let the OEM defaults stand; add hibernate as a battery safety net |
| Dual boot with Linux | **Fast Startup off**, hibernate off for the Windows volume |
| Server | Sleep off; hibernate usually off |
| Troubleshooting boot issues | Fast Startup off until resolved |

> **Caution: Fast Startup hides driver problems**
>
> With Fast Startup on, a "restart" and a "shutdown then start" are different code paths. If a problem appears only after shutdown, or disappears only after restart, Fast Startup is the variable — not the driver you just updated.

## Sleep that drains the battery

If the machine loses power in sleep (laptops with Modern Standby), see [Modern Standby](https://winbook.fyi/en/hardware/laptop/modern-standby/) for the `powercfg /sleepstudy` method. That is a different failure mode from hibernate.

## Related

- [Where the boot minute goes](https://winbook.fyi/en/windows/perf/boot-time/)
- [Power plans](https://winbook.fyi/en/hardware/laptop/power-plans/)
- [Modern Standby](https://winbook.fyi/en/hardware/laptop/modern-standby/)

## Lab boundary

E1. Power states and Fast Startup behaviour are documented by Microsoft. The lab VMs expose no real sleep/hibernate path, so no resume timings are published.

## Sources

- [Microsoft Learn — System power states](https://learn.microsoft.com/en-us/windows/win32/power/system-power-states)
- [Microsoft Learn — Powercfg command-line options](https://learn.microsoft.com/en-us/windows-hardware/design/device-experiences/powercfg-command-line-options)
- [Microsoft Learn — Windows Setup command-line options](https://learn.microsoft.com/en-us/windows-hardware/manufacture/desktop/windows-setup-command-line-options)

---

Source: https://winbook.fyi/en/windows/perf/sleep-hibernate-faststartup/