because if you don't use your computer the screen is turned off

if you still no using your computer the next minutes, then the computer change to stand by

 

if you don't want that your computer change to stand by, then in power managment (the battery in the corner of the clock)  select more power options, select yourpower plan, and then click in change plan setting, then there is the time needed to turn off your screen and to put your computer in stand by, change the stand by time, to never.

 

if you only don't like to close and open your computer, then use the power buttom, it will wake up your computer.

thanks!