System
Ubuntu MATE 15.04
Machine Toshiba Radius P55W-C5200
http://www.toshiba.com/us/computers/laptops/satellite/Radius15/P55W-C5200
External monitor
Lenovo LI2323S 23" LED IPS
uname -a
3.19.0-25-generic #26-Ubuntu SMP Fri Jul 24 21:17:31 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
The option in Power management
“Put display to sleep when inactive for” x minutes
does not turn off the monitor
Thanks
[EDIT]
Cable is HDMI
Does it sleep when connected to a different operating system/menu? What cable are you connecting with?
I’ve experienced such a wide variety of behaviour with different hardware, cable and OS combinations that I susect it has more to do with the monitor’s firmware than host computer OS.
Other OS/menu tries:
Display goes to sleep
Tried to install MATE manually
How do I install MATE (the desktop environment)?
Switching to MATE, the display does NOT go to sleep
[UPDATE]
This behaviour only happens when the laptop lid is closed. Keeping the laptop lid open, the monitor goes to sleep... still qualifies as a bug, I think, but it's a workaround.
Other than that, MATE look great, keep on the good work, MATE team 
.
Hola, tengo el mismo problema con ubuntu MATE 14.04 y 16.04 
no me pasa con Unity/Xfce/Mint.
Efectivamente me pasa cuando tengo conectado un monitor externo conectado y el portátil cerrado.
¿has encontrado alguna solución al respecto? es una pena no poder usar esta distro fantástica por este problema.
Un saludo y muchas gracias
I have the same problem with Ubuntu Mate 20.04 LTS, when my Lenovo laptop is docked into docking the station the external monitor never goes to sleep. It was fine with Ubuntu Mate 18.04 LTS.
