FMS itself has no sleep setting, so something else is going on. Does the machine go to sleep?
Describe what you see happening a little better. Also, what does the FMS event log say?
This could be an energy save setting in your system preferences (Mac … or equivalent if Windows). Make sure you set the computer or hard disk to never sleep; enable energy save for display only.
And you may have shut down FM Server then do a full reboot to be sure that the new energy settings take hold in the system. If seen that issue on some OS versions.