Question PC goes off in sleep mode.

Update your post to include full laptop hardware specs and OS details.

What is that "period of time"? Some fixed number of minutes?

Open Control Panel and go to Power Options.

Take some screenshots of the current Power Option settings.

Post the screenshots here via imgur (www.imgur.com).
 
Look in Advanced Power Options - ensure that the configuration settings therein are as you wish/require.
Many, many options.

Another place to look is Task Scheduler.

There may be some trigger in place that after "X" amount of time turns the laptop off.