Ever noticed that when people walk into their quarters on the Enterprise (ST:NG) they first thing they usually say is "Computer: lights". Why are they off in the first place? Does it matter at all if they just leave them on all the time (except to sleep, of course)? They are constantly converting matter to energy and back (replicators, transporters, etc.) On the Enterprise-D there are 400-odd people eating three replicated meals a day, not to mention all the other energy consumption... and yet they turn the lights out when they leave the room.