What is the hotkey to put my Windows to sleep?

In English Windows (versions prior to Windows 10)

Windows , → , → , s will put the computer to sleep.

In English Windows 10

Win+X , U , S

If sleep is configured to be hybrid, then it will be a hybrid sleep.

Windows doesn't have a standard hotkey or keyboard shortcut for going to sleep - except the "sleep" button that you see on some keyboards.

Instead, you can use a program to trigger the sleep state, and you can assign a shortcut to launch this program:

1. Download Steve's Wizmo tool.
2. Create a shortcut to this program on your desktop.
3. Right-click the shortcut, and select Properties.
4. In the field Target, add the word standby. *)
5. Click in the field Shortcut key, then press the key combination you want to use. This is then displayed in the field. Note: Not all shortcuts work, e.g. Win+S gets translated to Ctrl-Alt-S instead.
6. Click OK and try out the shortcut!

*) See the Wizmo webpage for complete documentation. Wizmo can do lots more!

You can use the free utility NirCmd as follows:

"C:\Program Files\Nircmd\nircmd.exe" standby

The rest you can find in torbengb's answer.

You can try this command instead (no external programs are required):

Rundll32.exe Powrprof.dll,SetSuspendState Sleep