The masks are unlikely to fall off. However, some people remove them during their sleep. This generally happens when they are not getting a sufficient supply of air. This type of problem can occur if there is a leak in the mask or the hose that supplies it. Leaks reduce air pressure making it harder to breathe. Leaking can also be a problem if the mask is not the right size for your face. When a mask becomes old and worn it can do a similar thing. As a rule of thumb, you should be replacing your CPAP mask every six to nine months. It’s also possible the pressure of the machine may need adjusting. If you wake up with your mask off and suspect it’s due to a problem with the machine you should tell your doctor at once.