What Is Sleep Paralysis? Causes, Symptoms and What to Do

You wake up – or at least, you think you do. Your mind feels alert, but your body won't respond. You can’t move your arms, your legs, or even your head. There is a heavy pressure on your chest, and in the corner of the room, or crouching on top of you, you feel something.

That is sleep paralysis. And as terrifying as it feels in the moment, there is a clear scientific explanation for it.

So, what is sleep paralysis?

Sleep paralysis is the experience of waking up, or coming close to waking up, while your body remains in a state of muscular paralysis. You are conscious, or semi-conscious, but you can’t move, no matter how hard you try.

It often comes with a feeling of pressure on the chest and can last anywhere from a few seconds to a couple of minutes. For some people it happens once and never again, while for others, it becomes a recurring pattern.

Sleep paralysis is more common than most people realise. While it’s not dangerous, it doesn’t make it any less frightening in the moment.

What is actually happening in the brain?

To understand sleep paralysis, you first need to understand why your body goes still during sleep in the first place.

The reason is pretty technical. Deep within the brain, in a region of the hypothalamus called the Ventrolateral Preoptic Nucleus (VLPO), your body manages the transition into sleep. The VLPO releases a chemical called GABA, an inhibitory neurotransmitter that essentially signals to your body that it is time to shut down and rest. That signal travels to the brainstem, which cuts the connection to your skeletal muscles. The result is a state called atonia – a temporary, intentional paralysis.

In simple terms it means that the chemical 'switch' in our brains switches off the signal to the skeletal muscles.

It’s important to note that this is not something going wrong. Instead, it’s your body's built-in safety mechanism. Without atonia, you would physically act out your dreams every night. Sleepwalking happens when the switch from movement to paralysis is delayed. Sleep paralysis happens at the other end of that same process.

What causes sleep paralysis specifically?

Sleep paralysis occurs when the timing between your brain waking up and your muscles switching back on gets out of sync. Your conscious mind becomes aware before the atonia has lifted. The body is still in its paralysed sleep state, but your mind is already active and trying to make sense of the situation.

That mismatch, as brief as it is, is what causes sleep paralysis.

Why does sleep paralysis feel so frightening?

The physical sensation of being unable to move is alarming enough. But sleep paralysis is also scary for something else – the hallucinations.

Often described as a ‘sleep paralysis demon’ or the ‘old hag’, many people report seeing a dark presence in the room, feeling a weight pressing down on them, or sensing that something is sitting on their chest. These experiences, although not real, do feel completely real at the time.

The explanation is rooted in the semi-waking brain state. You are not fully asleep, but you are not fully awake, either. In that in-between state, the mind is still capable of generating vivid sensory experiences, essentially dream-like imagery overlaid onto your waking perception of the room. The physical sensation of atonia, that heavy, immovable feeling, becomes the raw material. The brain interprets the inability to move as something holding you down, and fills in the blank with a presence.

What are the causes of sleep paralysis?

Sleep paralysis tends to occur during transitions in and out of REM sleep, the stage most associated with vivid dreaming. Several factors can increase the likelihood of it happening:

• Stress and anxiety: Theseare among the most common triggers, particularly for isolated or first-time episodes. When the nervous system is highly activated, sleep architecture can become disrupted and transitions between stages less smooth.
• Sleep deprivation: This makes irregular sleep transitions more likely. When you are overtired, your body can enter REM sleep more abruptly, increasing the chances of a mismatch between brain and body.
• Disrupted sleep schedules: Changing schedules associated with shift work, jet lag, or inconsistent bedtimes, interfere with the regularity of sleep cycles and can contribute to episodes.
• Sleeping on your back: This is also associated with a higher frequency of sleep paralysis, though the reason for this is not fully understood.

For most people, a one-off episode links back to stress or a run of poor sleep. If you address those, it may not happen again.

Why is it so hard to fall back asleep after a sleep paralysis episode?

When sleep paralysis strikes, your brain interprets the experience as a genuine threat. It doesn’t matter that nothing is actually wrong – the perceived danger triggers a release of wake-promoting hormones, including cortisol and adrenaline, which brings you fully back to consciousness.

Cortisol is useful in a real emergency. But in this context, it works against you. By the time the episode has passed, your stress response is in full activation, your heart is racing, and your body is primed for wakefulness. Getting back to sleep after that takes time, and for some people it feels almost impossible.

This is one reason sleep paralysis can become disruptive – even when episodes are short.

What should you do about sleep paralysis?

That depends on how often it is happening. If you have had one or two episodes and they appear tied to a stressful period or a stretch of poor sleep, there is often no need for intervention. Addressing the underlying cause, whether that is stress, inconsistent sleep timing, or exhaustion, may be enough to stop it recurring.

However, if episodes become frequent, or if they are significantly affecting your sleep quality and daily life, sleep paralysis is a medically recognised sleep disorder and worth raising with your GP. A sleep specialist can assess what is driving the pattern and recommend appropriate support.

The most important thing to know is that sleep paralysis, however frightening it feels, is not harmful in itself. In fact, your body is doing exactly what it is supposed to do. The timing is just slightly off.

If disrupted sleep is becoming a regular problem, it’s worth looking at the fundamentals. The right sleep environment, a consistent routine and a mattress that supports genuine rest can all reduce the stress and sleep deprivation that make episodes more likely. Browse our Sleep Journal for more on building better sleep habits.