What is the biological origin of the following behavior:

Sometimes, during the early, formative phase of sleep (when you have a gradually dimming consciousness of what all is happening around), you feel some kind of a tremor of sorts - a feeling as if you are about to fall off. The body responds by twitching the limbs, so as to "balance", and you end up waking up for a second or two, where you realize that you aren't about to fall and all is fine.

Since Cerebellum is involved with maintaining equilibrium and balance, I have the intuition that this is related to some transient behavior where the Cerebellum is dimming down. When you are in deep sleep, you do not have a sense of balance, so I guess Cerebellum does dim down the sense of equilibrium.

But the apparent flaw in my reasoning is - if this was indeed the origin, one would have experienced these tremors every time one slept. That doesn't happen, it is only occasional (AFAIK).

So, biologically, what is the true origin of this behavior?

  • 1
    $\begingroup$ FWIW, I am aware of the fact that answers on SE are well-intended to have a broad scope, but still, I feel necessary to point out my familiarity with Biology is quite basic and limited to as much as is taught in a typical high-school (to people who take up Biology as a chosen stream). Hence, I'll request the answerer the please point me towards some introductory literature, in case using more advanced terms is unavoidable. Thanks :) $\endgroup$
    – 299792458
    Commented Apr 25, 2015 at 7:50

1 Answer 1


These are called hypnic jerks, also known as hypnagogic jerks or sleep starts. They are normal part of the wake-to-sleep transition. Hypic jerks consist of non-periodic myoclonic movements, generally involving an isolated limb.

The exact physiologic origin of hypic jerks is unknown. We can say that they correlate with a particular waveform on the EEG known as a K-complex:

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Picture from http://en.wikipedia.org/wiki/K-complex

K-complexes are a normal part of Stage II sleep and can evoked using auditory stimuli, even if sleep is not interrupted. In the case of hypnic jerks, the K-complex is usually followed by alpha (wake) activity. To my knowledge, there is no evidence that hypnic jerks originate in or are routed through the cerebellum. K-complexes themselves are related to cross-talk between the cortex and the thalamus, a circuit that doesn’t involve the cerebellum.

In addition to the EEG findings, it has also been shown that the jerks are accompanied by autonomic activation — transiently increased heart rate and respiratory rate. There is also an (unexplained) subjective aspect. You described it as feeling "as if you are about to fall off,” which seems to be consistent with the published literature on this. As mentioned in the review cited below, some people report a sensation of "falling into the void.”1

The clinical implications, in case you were wondering, are summarized well by Vertugno et al.:

Sleep starts are common physiological phenomenon affecting up to 70% of the adult population and their course is benign, resolving without any neurological sequel. Therefore, reassurance and counselling are all that is needed to treat the patient. Though an adequate explanation and reassurance may be sufficient, some patients may require a small dose of clonazepam (0.5–1 mg at bedtime) to ameliorate the symptoms on a short-term basis.

1. Vetrugno R, Montagna P. Sleep-to-wake transition movement disorders. Sleep Med. 2011 Dec;12 Suppl 2:S11-6.

You requested in a comment a reference to introductory material on the topic. What we know about the neuro-physiology of sleep is mostly based on the analysis of EEG (brain-wave) tracings. I therefore refer you to this introduction to sleep polysomnography if you would like to get a better handle on how scientists think about sleep.

  • $\begingroup$ Thanks for your response. I have +1'd it and will go through the linked articles a bit later (a bit short on time right now), and get back to you if I need any help in understanding them. I'm deferring the accept for until then, but I'm sure this is only temporary. Thanks again :) $\endgroup$
    – 299792458
    Commented Apr 25, 2015 at 13:48
  • $\begingroup$ @TheDarkSide Unfortunately, the Vetrugno paper requires access through a library (or paywall) for viewing the full text. It was the only review I found that includes significant discussion of the topic in a relatively recent issue of a top-tier sleep journal. If you're not able to access it, let me know and I can see about finding something else. The PDF of the polysomnography chapter should be viewable in full, though. $\endgroup$
    – Susan
    Commented Apr 25, 2015 at 14:00
  • $\begingroup$ I do not think sleep starts are associated with sleep starts. K-complexes are a characteristic of sleep (Stage 2), although they can be seen is deeper stage 3 and 4 sleep as well. Sleep starts tend to occur during the wake-sleep transition. Sleep starts can also be "Hypnagogic hallucinations" which can vary to sensations of falling (common) to loud noises leading to the infamous "exploding head syndrome". Sound is not the defining characteristic of Hypnagogic hallucinations. ;) See sleepassociation.org/patients-general-public/… $\endgroup$
    – Panther
    Commented Apr 25, 2015 at 14:08
  • $\begingroup$ Sleep starts can be exacerbated by caffeine and "stress" . See also - sleepeducation.com/news/2013/08/22/sleep-starts $\endgroup$
    – Panther
    Commented Apr 25, 2015 at 14:09
  • $\begingroup$ @bodhi.zazen Re. your first comment - you’re confusing a lot of things here. 1) Yes, K-complexes are a defining characteristic of Stage 2 sleep, but they are also frequently associated with brief arousals (alpha frequency), i.e. sleep-wake transitions. 2) The literature about auditory-evoked K-complexes within sleep is extensive. 3) Hypnagogic hallucinations are REM-sleep phenomena, most often seen in narcolepsy, sometimes in sleep deprived young healthy people. Totally different deal. 4) Exploding head is different still, a type of HH but likely different mechanism. $\endgroup$
    – Susan
    Commented Apr 25, 2015 at 14:22

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