Something is not right.
Yes. Laser printers have a sleep mode. They power down the fusers because they consume a lot of energy.
However, they should NOT power down the network interface. When a job is submitted to the printer (over the network) it should wake the printer up.
I'm having the same problem TheCamba20 is talking about: the printer goes to sleep but does not wake up when a job is sent to it over the network.
We have two of these printers (C1765fn) which we just purchased last week. If Dell does not have a fix for this problem then we are going to return them.