First place to start is a firmware update. Navigate out to the drivers and downloads area for this printer and make sure you are up to date.
Aside from that I would agree that it is a sleep issue. Something needs to go out and wake up the printer before it is able to print for the day. My guess is that it recieves enough activity through the day to stay awake enough until everyone goes home.
Not sure if there is a real fix for wake up problems outside of swapping the printer for a different model. You might want to look into a timed power adapter that is programmed to power cycle every morning before everyone arrives. We have a few printers that do this every morning for other reasons and I can see that situation playing well for you in yours.
You could also look into a script that runs on a machine over night to ping the printer and keep it active. Not sure if PING would be enough by itself, you might need the assistance of a scripting service like AutoIT to navigate out to the Embedded Web Server (EWS) and click Apply for you once an hour. You would need a server or work machine connected to the network overnight to facilitate that option though.