Guest post by Linda Cotrina: Imagine feeling like you were awake but somehow unable to move. You feel an immense pressure on your chest, as if someone or something is holding you down. You attempt to move, perhaps even shout, but nothing. You are frozen.  This is an all too familiar experience to me, something I've come to identify through an ample amount of research as sleep paralysis. What is it exactly? Well, in the most basic terms, it's "when you wake up feeling paralyzed and you're unable to move your arms or legs," says Dr. Brian Murray, a neurologist and sleep specialist at the Sunnybrook Health Sciences Centre in Toronto. Dr. Murray states that sleep paralysis occurs when our motor system and our cortex go to sleep at different rates. When this happens "you can perceive this inability to move," says Dr. Murray. What makes it such a frightening experience is the feeling that there is "a presence" watching you and at times you feel that it is preventing you from being able to move or speak, a feeling that I have often had when this phenomenon has occurred. I've felt as though I was going crazy, but thankfully there is a scientific explanation to this feeling of a "presence." Dr. Murray says it's an issue with the "control of the sleep-wake state." It takes about 90 minutes to go into dreaming sleep or rapid eye movement sleep (REM) but at times we can enter REM sleep as soon as we hit the pillow, he says. As we are in this dreaming sleep state, our bodies remain paralyzed and "sleep paralysis may reflect some pieces of the dreaming state; the not being able to move, plus the dreams which are like hallucinations," says Dr. Murray. Scary right? I know, but although it can be terrifying, thankfully, sleep paralysis isn't considered to be a serious health problem. Here are five things to know about sleep paralysis: 1) Sleep paralysis can occur at one of two times: when you are waking up (known as hypnopompic) or when you're falling asleep (known as hypnagogic). 2) It can occur as a result of sleep deprivation and is often associated with a rare disorder called narcolepsy, says Dr. Murray. Anxiety, depression and alcohol use can also be contributing factors to the occurrence of sleep paralysis. 3) According to a study done by the University of Waterloo, 25 to 30 percent of the population report having experienced a mild form of sleep paralysis at least once and 20 to 30 percent have experienced the phenomenon on numerous occasions. Dr. Murray states that about 50 percent of people experience sleep paralysis in their lifetime. 4) The study done by the University of Waterloo reports that an episode of sleep paralysis can last seconds or in some instances longer. As sleep paralysis is occurring, it is usually perceived to be longer than it seems, says Dr. Murray. 5) Sleep paralysis is sometimes referenced as incubus attacks and in some cultures is referred to "old hag," a term that came from a superstitious belief that a witch or old hag sits on the chest of the victims, immobilizing them. Have you ever experienced sleep paralysis?  Photography courtesy of thinkstockphotos.ca.