As long as it's in a well ventilated area, there's no danger. At least, no more danger than there would be if you left your desktop computer on all day. The danger to laptops is overheating. Because all the hardware is in such cramped quarters, it tends to get very hot. The bottom gets hot, so be careful where you put it. You may even want to put it on something raised so the air can get to the bottom. If you leave a laptop on blankets or a bed, it will often overheat because the insulation of the material is keeping the heat in. A tip that may help you when you're transporting your laptop is to take out the battery to ensure that you don't accidentally leave the laptop on.