Have you ever woken up not knowing where you are? Do you wake up feeling confused or panicked, unsure of how you got there or how long you've been asleep for?
Sleep expert Emeritus Professor Leon Lack, from Flinders University, explains what causes the strange sensations we experience when waking up.
Why do I wake up confused and disorientated?
Professor Lack said while there hadn't been much research around the phenomenon, the body's "rollercoaster" sleep cycle was probably to blame for feelings of confusion and disorientation.
"Our sleep is like a rollercoaster going through 90 minutes of sleep cycles, starting in deep sleep and then light sleep … going across the night," he said.
"That deep sleep stage is the period where the conscious part of the brain — the upper part of the brain — is least activated.
"If you wake up and are quite confused as to what time it is, where you are, or who your friend is that just woke you up, you're likely to have woken up out of the deeper stages of sleep [which] tend to occur early in the night.
"If you're a parent and your infant starts to cry or if your bed partner is a very heavy snorer … then that's the time that you'll experience that sort of confusion.
"It might take you several seconds to orientate yourself in the world."
Are these common sensations?
Professor Lack said most people, at some stage, experienced odd sensations during sleep or while waking up.
"It would be experienced quite a bit, on a mild level," he said.
There are some sensations, like muscle paralysis, that are far less common.
"That's another phenomenon, but caused probably by the opposite [stage of sleep] — the REM sleep, as opposed to the deep sleep," Professor Lack said.
"During the REM sleep, your brain is highly activated with increased blood flow and increased processing of information and [it uses] internal information to create dreams.
"Your body is [also] actively paralysed — this stops you from acting out your dreams because you wouldn't want to run out of the room if you thought you were being chased by a burglar in your dream.
"But that muscle paralysis can carry over occasionally when you wake up out of REM sleep.
"Your eyes are open and you can look around the environment, but you realise you can't move anything else in your body and you feel paralysed.
"It is a benign event that usually doesn't signify anything going wrong."
So why do we have a sleep cycle?
Professor Lack said the brain's sleep cycle — comprised of deep and light sleep stages — had probably played a role in our survival as a species.
"[Most species] have this sleep cycle — it may have served some vigilance-type function," he said.
"Every 90 minutes or so, you come into light sleep and you're much more responsive to the environment [and] just awake for a few minutes to check out that everything's still OK, and that there's no danger.
"Having that type of sleep pattern across the night may have helped to increase our chances of survival.
"If the sabre-toothed tiger came into your cave at night and you came out of deep sleep, you would probably respond more slowly.
"If our whole sleep period were deep sleep, then we would be more vulnerable to any [environmental] changes that may occur."
Professor Lack said anyone who experienced unusual or unsettling sensations more frequently, or more severely, should see a doctor.
"If it becomes a much more regular phenomenon then I would seek some advice from a medical practitioner, who might refer you on to a sleep clinic, or it may be indicative of something else in rare occasions."