Ever awaken with a start just as you're drifting off to sleep? Studies suggest that 60 percent to 70 percent of us experience these "hypnagogic jerks" from time to time. But what causes these strange episodes, which are sometimes accompanied by a feeling of falling?
No one really knows. But that hasn't stopped speculation.
One longstanding hypothesis holds that hypnagogic jerks (also called hypnic jerks or sleep starts) are an evolutionary adaptation left over from a time when our prehuman ancestors lived in trees.
The idea is that awakening with a start, just at the onset of sleep, helped protect these tree-dwellers from injury or death by helping them avoid falling from their perches when they nodded off.
Not everyone is convinced.
"I cannot imagine that there is a shred of scientific data to support this wild speculation," Dr. Mark Mahowald, a retired sleep specialist at the University of Minnesota, told The Huffington Post in an email. If it did have any evolutionary basis, he added, one would expect it to occur most of the time -- not just occasionally.
Dr. Jerry Siegel, a professor of psychiatry at UCLA, echoed Mahowald's assessment of the hypothesis, saying in an email to HuffPost, "I think that's a stretch. The hypnic jerk that I remember from my childhood almost caused me to fall out of bed, although one could argue that it was my almost falling out of bed that triggered the jerk."
Mahowald ascribes hypnagogic jerks to a glitchy interaction between different phases of sleep.
"As we fall asleep, we typically enter non-rapid eye movement (NREM) sleep," Mahowald said in the email. "Some have suggested that sleep starts may represent a snippet of rapid eye movement (REM) sleep that sneaks in during the transition from wake to NREM sleep."
Another prominent sleep expert sees the jerks as evidence of a neurological tussle between systems in the brain that promote sleep and systems that promote wakefulness.
"One of the things that happens as you fall asleep is your muscles relax, but the awake part may still be stimulating enough that it will temporarily overreact and you get this jerk of muscle activity," Dr. Carl Bazil, director of the sleep disorders center at Columbia University Medical Center, told New York magazine last year.
Whatever the cause, experts agree that hypnagogic jerks -- though sometimes scary -- are harmless.
Now you know.