There are number of causes of sleep paralysis. Sleep paralysis occurs when you are immediately woken from sleep whilst you are experiencing a dream. The paralysis sets in when the transition between the two states is not a smooth process as should be the case. The dream that you were experiencing becomes real for you and when you try and move you find that you are unable to.
This is because as far as the body is concerned you are still dreaming. So normally anything that will interfere with your normal sleep pattern will cause sleep paralyses. There are many things that interfere with the brain and bring it out of sleep. Light and sound are the two most common things.
When you go to sleep make sure the room is dark and there is no light filtering through to your face. After you've fallen asleep it is this same light that will cause you to be awoken suddenly. Most people also tend to go to sleep with the TV or radio on. The sounds produced by these appliances will definitely wake you before the period of normal sleep ends. Avoiding these two causes of sleep paralyses should decrease the episodes.
Other known causes are an inconsistent sleep patterns. It would really help a lot if you maintained regular sleeping hours. Make it a point to go to sleep and wake up the same times everyday. This way the body becomes conditioned to the routine.
Nicotine, alcohol and tobacco are known stimulants that interfere with your body's ability to stay asleep. Try and avoid them during night times. Make sure that you empty your bladder before you go sleep. As you can see there are many causes of sleep paralysis. There are also ways in which you can prevent it or avoid altogether.