Sleep mode is how many hardware manufacturers complete thier energy star criterea. That means when a device is not in use the device will lower its power consumption and hibernate/sleep until it is needed again. Disabling sleep mode is not advised.
Instead you should be looking into ways to configure your printer so that the automatic maintenance features of your network do not remove it due to inactivity. For example configure your printer to use a static IP address. You would also want to update your firmware to the latest version as a standard troubleshooting step. Finally, experiment and see if the problem is reproducable over WiFi or Ethernet Cable. If your network learns to get along with your printer your offline problems will go away.