Environmentally (Un)Friendly Business

I work for a rather large company whose PC's must number well over a thousand at a guess. Bearing in mind that we're all supposed to be doing our bit towards saving energy, why can't we turn them off at night? We are all told to leave them on (just turn off the monitor), so they can send updates out overnight (apparently if they send them out during the day, it will slow everything down too much), but I really don't know what updates they are talking about, maybe the odd Microsoft security update, but I can't think of much else. Even at weekends or when we are on holiday, they tell us to leave them on! It must waste the company thousands or even millions of pounds every year!