Devices left in sleep mode, are not turned off. They waste about two power stations’ worth of electricity each year and produce the same amount of carmon dioxide as 1.4 million long-haul flights or the climate impact of about half a million flights!
When you switch a modern electrical appliance off, it rarely actually shuts down: this only happens if you unplug it or turn the socket off. Some well engineered devices draw just milliwatts of current in standby: whereas others continue to suck up 30 watts from the mains, or 113 kg carbon dioxide per year without it doing anything.
from: How Can I Stop Climate Change? Friends of the Earth.