A condition known as sleep paralysis has been found to be relatively common in a group of several hundred Irish students.
Sleep paralysis is a temporary inability to move or speak which happens when someone is stirring, or less commonly, falling asleep.
They are awake but the body is briefly paralysed for a few seconds or several minutes before returning to normal.
Sleep paralysis does not harm, but for those who experience it the inability to move can be terrifying.
Jonathan Egan, Ruth Howard, and Kiran Sarma of the National University of Ireland in Galway, looked at what might be causing students to endure sleep paralysis and unusual sleep experiences.
They asked 400 students to fill out a survey on their sleeping experiences and presented the findings to the annual meeting of the Psychological Society of Ireland.
They discovered that sleep paralysis was found in a quarter of all students who took part in the study. It was associated with a history of a panic disorder, illicit drug use (ecstasy and speed), cigarette smoking and the consumption of more than 20 units of alcohol per week.
"Most individuals experienced sleep paralysis while lying on their backs. A significant majority of students with sleep paralysis reported sensing the presence of an intruder in the room with them; two-thirds reported feeling an incubus, or something or somebody on their chest, being smothered, strangled, or being unable to speak."
Many individuals also reported a sensation of falling during their sleep paralysis experience, said the authors. Half of participants reported experiencing a false awakening, and roughly 40pc had a sense of floating during their sleep paralysis also.
"Unusual sleep experiences were found to be negatively associated with agreeableness and consciousness. Individuals with more unusual sleep experiences are typically less compassionate and self-disciplined, and more suspicious and spontaneous than individuals with less unusual sleep experiences."
They pointed out that the study identified clear personality, health, and lifestyle factors that are associated with both sleep paralysis and unusual sleep experiences.
Experts say certain factors make people more likely to experience sleep paralysis, including sleep deprivation, irregular sleeping patterns and being a teenager or young adult.
It can also be a symptom of the sleep disease narcolepsy and there could even be a family link.
The advice is to try to have a regular sleep schedule, and not to smoke or drink before going to bed.
Antidepressants can also treat severe sleep paralysis.