Thank you for your inquiry. But I am afraid that the option to sleep can be disable for several options.
Also which model of Dell system do you have and which Windows Version did you install.
But some of the things I will recommend will be:
- Uninstall your Graphic driver.
- Reinstall your chipset drivers.
- Reinstall your Graphic Driver.
- Run the Windows Updates
- Verify your Device Manager for any missing drivers.
Also if you like send me the information and also a print screen from the device manager.