Almost everyone loves sleeping. Heck, it's one of our favourite activities. Soothing, lulling sleep with dreams of Candyland till the irritating alarm makes you wanna slap it to the wall.

Now imagine this scenario. 

You're tucked in for the night and suddenly, just like that, you're awake in the darkness, your eyes are stark open but unseeing, you don't know what's happening and your brain won't snap out of it. You can't move, not even a toe wiggle but you have the feeling that someone is there lurking in the shadows, observing your helpless state while you try to move your lips to make a sound, call for help, but you just lie there with a pressure on your chest, as if someone is sitting on it and you can't get enough air and you keep seeing that black figure. 

So something like this.

Source: reddit

There are accounts of sleep paralysis that date as far back as the 10th century in Persian texts and was originally considered as the nightmare. 

Myths and folklore contributed to the belief that sleep paralysis occurred due to visitations by demonic entities often with malicious intent or by incubi. The demonic presence was said to have sat on the chests of the victims thus leading to the suffocating sensation or the pressure on chest.

 While some still believe in these theories governed by religious beliefs, science has some perfectly reasonable explanations for the phenomenon.

Source: kitsapparanormal

Here are some terrifying accounts of sleep paralysis by people. Reddit user FLAPPY-BIRCH narrated the experience:

The way that it started for me was I woke up in the middle of the night and went into my kitchen to get a cup of water and when I looked into my living room there was like a hooded old person. IDK how to explain it really but I got super freaked out and instantly knew what was happening but it was so real I kept doubting myself. So I paced back to my bedroom and went back to bed, but when I woke up I was in your "typical" sleep paralysis episode. I was laying in bed, couldn't move, and there was a big black thing hovering above me. It almost felt like when your arm falls asleep, except your whole body. I kept forcing myself to go back to sleep but whenever I woke up the same thing happened.

Another experience by whatwouldscoobydo will leave you hoping this never happens to you:

I had it happen to me, in my early teens. I had no idea what sleep paralysis was at the time. I never realized I was paralyzed, always too focused on what I felt / heard / saw. I legitimately thought my house was haunted, or that I was somehow aware of other creatures others couldn't see (even though I knew logically this made little sense, I didn't know what else to attribute it to at the time). The one in particular was a dark humanoid figure that crab-walked (that was it's normal stance). It had no eyes or mouth. It would come up behind my head board and watch me, or crawl on to the walls and observe me. I never got the sense that it wanted to harm me, only that it was an observing predator and could kill me if it wanted to.
Source: mourningtheancient

This account by Terakian is plain freaky:

I had it happen about two years ago. Didn't see anything either. But I remember waking up and already feeling very afraid. Tried to scream to wake my girlfriend next to me, but couldn't - THAT'S a scary feeling. The unique aspect of it was that I remember when I tried to close my eyes and just go back to sleep, the room really felt like it was closing in on me - a loud and strong groan (like a building falling inwards) in my ears, and a powerful pressure on my chest. Only lasted around a minute, but honestly one of the scariest minutes of my life. Had no idea what was going on.

Sleep paralysis is dependent on the stages of sleep you're at. 

If it happens as you're falling asleep, it is known as hypnagogic or predormital sleep paralysis. 

When we fall asleep we gradually lose consciousness and our muscles relax and loosen up but sometimes during this period the brain remains aware and doesn't shut down and that's when we experience sleep paralysis where we are unable to move or speak but are aware.

Source: wikipedia

It commonly happens as we are waking up and this is called hypnopompic or postdormital sleep. 

This occurs because during sleep we shift through different stages such REM (Rapid Eye Movement) and NREM (Non Rapid Eye Movement) sleep. During NREM stage, we sleep deeply, this is when the body does its rejuvenating by relaxing muscles. This is followed by the REM stage where our brain has similar activity as when we are awake. 

REM is characterised by rapid side to side eye movements and detailed, vivid dreams but the body remains in lockdown mode to prevent us from acting out these dreams as they could prove dangerous. Sometimes our brain becomes aware before we slip out of the REM phase and that's when we cannot move or speak.

There are explanations for the hallucinations or sightings of dark figures during sleep paralysis. The feeling that someone may be in the room with you or sitting on your chest is due to the muscle paralysis that occurs in sleep. You take shallow breaths during muscle paralysis but when you try take deeper breaths you feel that you can't. The mind may interpret this as being strangled or as someone sitting on your chest.

Source: wondergressive

When we are in a state where we feel there is danger, such as in cases of sleep paralysis, the body goes into fight or flight mode and releases a surge of adrenaline. The heart beats faster and we are in a super alert state and so anything might seem more than it actually is. The brain might try to pinpoint dark shapes as potential danger.

We might even experience hypnagogic hallucinations where all our senses are stimulated to create something which isn't there. You might feel as if something is touching you or notice an unpleasant smell or see dark figures but that's just the result of a very creative brain. Our keen state during sleep paralysis makes our brain frantically run through possibilities of danger and our defense mechanism makes it seem that there is a menacing presence in our room.

Source: cbsnews

Sometimes our dreams overlap reality and when we wake up from vivid dreams we might feel for sometime that the presence in the dream is there for real.

Normally people go through sleep paralysis just a few times, commonly during teenage but these require no treatment. However, in case these are regular occurrences which are understandably terrifying and worrisome, you could: