Laptop shuts down after a minute with an external monitor?

So here's the deal - i have connected an external ASUS VG236 23" monitor to my Lenovo Y520 laptop over HDMI. The problem is that, if I don't interact with the computer for 1 minute, it goes to sleep, which is annoying. Doesn't happen when no external monitor is connected, at least i haven't noticed.
Any help will be much appreciated.

If you can get into the power options in the Windows control panel, you may find that it has different sleep settings for when it's being used with an external monitor compared to the internal one. Hopefully, this has just been set up incorrectly as the usual default would be the opposite of what you are seeing!

You may also find the monitor itself also has power saving settings

Guess you shoud increase Screen Saver time

Go into the Control Panel and the Power options icon and check what the timeout is there.

Also if you are running an external monitor with the laptop on battery that is going to draw a lot more battery power and could be triggering the power saving of putting the monitor to sleep.