If you don't realise it's happening, you can find yourself tightening the mask for the wrong reasons and causing more leaks.
Many masks are designed to manage a certain level of tube drag. With the machine turned on, pull gently on the tube to find out what tube drag feels like. Roll around a bit in bed to see if your tube gives you enough room to move, and how much tubing you actually need. Most masks have longer tubing available if you need it.