I get a lot of random questions from friends and relatives. It is an occupational hazard. This one just came down the grapevine:

Do you know how sometimes when you're lying in bed, starting to fall asleep and all of a sudden it feels like you're falling?

What does that mean?

What you are talking about is a benign "condition" called a sleep start or a hypnic jerk. I hesitate to use the word condition because these are incredibly common -- around 70% of people have these.

What happens is that the whole body jerks one or two times right before sleep onset or during light sleep. This is often accompanied by a feeling of falling or a sensory flash -- like lights or a tingling sensation. While they are common in people, they are more common in people with Restless Legs Syndrome (RLS) or people who are sleep-deprived.

The relevant review by Walter (2007) had this to say:

If only one or two whole-body jerks happen occasionally just before sleep onset, consider the diagnosis of sleep starts (also called hypnic jerks). This is common and has been experienced by almost everyone at one time or another. Only on occasion are the hypnic jerks excessive and bothersome to the individual. If the jerks are excessive prior to sleep, the differential diagnosis includes the periodic and aperiodic involuntary movements that are relatively commonly seen in patients with RLS. In a minority of patients with RLS, these movements may be more prominent than the leg discomfort and may be very rapid or myoclonic in speed. In some cases, these very rapid movements in RLS patients during wakefulness electrophysiologically show the characteristics of propriospinal myoclonus, which is initiated by nerve fibers of spinal cord origin. Propriospinal myoclonus during wakefulness in the absence of RLS and PLMS must also be considered in the differential of sleep starts (hypnic jerks), but this appears to be a much less common disorder. According the new International Classification of Sleep Disorders, sleep starts (hypnic jerks) are characterized as follows: the patient complains of sudden brief jerks at sleep onset, mainly affecting the legs or arms; and the jerks are associated with at least one condition from among a subjective feeling of falling, a sensory flash, or a hypnagogic dream. In addition, the disorder must not be better explained by another sleep disorder, medical or neurologic disorder, mental disorder, medication use, or substance use disorder.

Sleep starts are a normal phenomenon occurring in persons of all sexes and all ages with a prevalence of 70%. They presumably arise from sudden descending volleys that originate in the brainstem reticular formation and are activated by the instability of the system at the transition between wake and sleep. Only occasionally do they become excessive, and, in most cases, reassurance that this is a normal phenomenon is all that is needed. There are no known treatments. (Emphasis mine. Citations removed.)

The cause of hypnic jerks is not entirely clear. During sleep onset, a part of the brain called the reticular formation sends a signal down the spine that causes your muscles to relax. (Actually, if I understand correctly, it causes a change in response of motor neurons from activation to inhibition. This is called the "reticular response reversal" and has been the subject of considerable research.) The theory is that occasionally this signal to relax is misinterpreted by the muscles as a signal to contract causing the spasm.

We also know that there is a measurable burst of activity on the EEG during hypnic jerks. This was shown way back in 1959 by Ian Oswald himself. (Oswald is sort of a big-wig in the area of sleep research. OK, fine. Some of you may have heard of him.)

The following is an EEG trace from Oswald's 1959 paper on the subject (Figure 2):

The arrows indicate the jerk in the leg leads -- RL = right leg and LL = left leg. (Yes, I recognize it is hard to see, but it isn't a very big jerk.) RS is the respiratory trace. The numbers are the EEG leads. ECG is the heart trace.

So the take home message would be that what you are experiencing is totally normal. If it becomes a regular problem, discuss it with your doctor.

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Incidentally, any other neurology or neuroscience questions are appreciated and will be answered if they are interesting and if I am not to busy at work that week. You have my email in the Contact section.