The reason nobody has come back with an answer as to what "it" might be is that nobody knows. Our doctors don't even know. All we have are some educated (or not) guesses. I will share my information, summarize what we seem to know about the shivering syndrome so far, and suggest how we might contribute to finding out more. For now, however, we'll see that the common thread is nerve damage, and the way to treat it is with neural protective medications such as depakote. I suggest that the source is chronic obstructed breathing at night.
MY CASE:
I get intense shivering sometimes when falling asleep, mostly when waking up (usually it wakes me up), even though my room temperature is controlled at a constant 70 degrees (variance of about 2 degrees). With or without the shivering I nearly always wake up freezing, disoriented, and weak.
The shivering is actually so intense that it hurts, and the only way I have got it to stop is to apply sudden and direct pressure to pressure points, which results in a "charlie horse" or "dead arm". My right side is affected more so than the left, but both are involved. By the time it stops I am usually exhausted, then pass out, and miss work/class. But I have a very sensitive sleep schedule such that any changes to a normal routine throws me off for several days, so the consequences extend beyond the nights I'm affected.
On days that I do not get the violent shivering, I am still freezing. If I take a very hot shower, I can "burn off" the chill (too long a shower and it makes me feel sick though). If I do not get a shower, I can feel chilled until the, sometimes late, afternoon.
I have a mental health history of ADHD and hypersomnolence (a waking up problem) up until I incurred a brain injury, after which I had many more problems, such as migraines, (something like) bipolar, and this shivering thing at night. Sleep apnea was diagnosed after my injury as well. (right side of body affected more -> left side of brain; I sustained my injury on the left side, which affected language).
DATA:
I have done some investigation on my own trying to pinpoint the problem(s), a sleep study at a sleep lab, as well as generating my own data. If I had chronically low body temperature in the morning, that would be an indication of thyroid disregulation, but my peripheral skin temperature, continuously recorded every 4 seconds through the night for several nights, was not abnormal. I experienced a decrease in symptoms when I restarted valproic acid (like Depakote).
LIMITATIONS:
I have not yet kept a journal of my meals, meds, and their times, nor the times I slept. I do not have a "low light" web-cam, which could record video of my sleep, to control for, say, sleeping on the arm the probs are attached too, and documenting any nocturnal behavior that may interest my doctors.
RESULTS:
My information did not shed light on the problem other than it may be related to my brain injury or sleep apnea - though limbic structures (sleep, breathing) are very deep & well protected areas of the brain.
SYNOPSIS:
So far I have seen posts about this problem (or similar) from people with histories of bipolar, anxiety, MS, epilepsy, ADHD, head trauma, depression, (unspecified) personality disorder, and perhaps (pre)diabetes. The classes I have read about seems to include 2 distinct "types" of the syndrome, namely, those who experience shivering with freezing, and those who just experience the shivering. My guess is that there will be two separate etiologies involved. Since folks seem to have a common set of diagnoses, the syndrome may be related to meds or the psychological diagnoses themselves. Sleep terrors, for example, can cause people to wake sweating, though I'm not sure about the tremor. As suggested by others, the syndrome(s) could be related to medical conditions such as diabetes, MS, psychological diagnoses such as bipolar, or anxiety, or nerve damage - such as caused by blunt injuries or medications. The folks without a mental health history, which does not mean they do not have something that was not yet identified, tentatively indicate that a mental health history is not necessary. Of the meds, Lexapro and Lamictal have been implicated with increased incidence, and anti-seizure meds used in bipolar have been implicated with a decreased incidence.
IMPRESSIONS:
Since this shivering syndrome has occurred in people both with and without mental health histories, and since those with mental health histories comprise a large cross-section of diagnoses, I doubt psychological diagnoses are the cause. However, one must be cautious to properly rule out night terrors, perhaps sleep apnea, and certainly panic disorders (difficult, because the shaking syndrome itself can be scary) and a history of trauma. In addition, one must keep in mind that many of the people with mental health diagnoses presented very complex histories. For people with diagnoses, the that complexity itself may be an indication. Even so, I suspect the shivering syndrome could be treated separately from, e.g. trauma, even if trauma somehow caused it. The implication of several SSRIs suggests that seretonin syndrome, other side effects, and/or withdrawal (tapering off, & morning dose wearing off at "troth" levels 12 hours later). That anti-seizure meds have been implicated with symptom improvement may suggest an underlying seizure disorder, or a withdrawal syndrome experienced at troth levels (if taken once a day, and not long acting formulations), or that it is acting as a protective factor for nerve damage. Folks with a family history of diabetes experiencing shaking suggest blood sugar, or other blood level "stuff", such as glucose, may be important factors. The implication so many different things, that of MS is rather sobering, suggests this shivering syndrome may be the direct result of nerve damage, which can be caused by direct injury (car accident), MS, diabetes, the meds for the mood and anxiety disorders, some heart conditions, sleep apnea, and trauma (even without personal injury/insult). On the other hand, perhaps these various diagnoses & conditions somehow make us more subceptable to the shivering. The best indication for treatment seems to be (though limited, as seen in the case of the post of MS & another one I saw about a person with epilepsy) meds that act as neuroprotective factors such as the anti-seizure medications. The most immediate suggestion for a related complication, given that shivering is a limbic response related to other limbic system functions such as sleep and breathing, is that you snore (perhaps related, but not necessarily causal, though Apnea can cause nerve damage if it is bad/chronic enough).
FURTHER DIRECTIONS:
Trauma may somehow "turn on" this shivering response, as it is no doubt capable of influencing sleep. We need shiverer's to supply (not detail but the fact of/kind of) whether we have - as a group - a history of trauma. Insult to the brain could be another source, in particular, left temporal regions & perhaps limbic structures. Those with brain injuries need to supply the nature of their injury compared to at least the side of the body they tend to experience shivering with more. A more frequent thread for a possible etiology is that of (type II) diabetes. Journals with food intake should help clarify whether shivering is related to blood sugar or glucose (or not). Whether or not recent sugar/glucose levels were elevated at the time of the shivering would be useful (within a week, leading to the test, perhaps). I am very curious about the nerve damage theme I presented, which, admittedly, doesn't tell us why or where it would be happening (except in the case of known, non-diffuse injuries). Perhaps it is related to the limbic system, and either caused by or related to sleep apnea (or some other limbic function). The only way to tell is to gather more information. Sleep journals will be the best tool. If you do have apnea, please journal when you wore a CPAP/BIPAP mask (or not) or wore a dental device, related to your symptoms. If you're on meds, journal times/doses. Journal exercise, too. No reason why we cannot crack this thing.
Sincerely,
KeithC.