You need to login as an admin ( hold down the shift key while powering on or loggingoff ) and disable the Enhanced write filter ( the padlock icon on the system tray ) which would require a restart and after which you can go to the Power and Sleep options and change the options according to the settings you want.
Please check this link aswell: https://social.technet.microsoft.com/Forums/windows/en-US/34ab1111-39a3-4e3c-82a7-7fbeb4af8e79/disable-win-8-idle-timeout?forum=w8itprosecurity
Regards
SATISHP4
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