Many people know the sudden, unexpected and uncontrollable shudder that usually travels from the shoulders down the back like an electric shock. The upper body visibly shakes for a second without any discernible reason. Where does this shudder come from?

It is not known exactly what causes these sudden, brief shudders, but there are various theories. Some people experience these shudders immediately after urinating, others just before falling asleep and still others completely randomly under a variety of circumstances. The fact seems to be that no external influence is responsible for the uncontrollable shivering, so neither does it occur exclusively in particularly cool or particularly warm weather, nor is there a pattern of any change in the environment that could be responsible for the shivering. 

What is certain, however, is that it is a bodily reaction in the central nervous system that causes certain muscles to contract suddenly and thus trigger these spasms.

As this shuddering occurs very frequently, there is no shortage of superstitious explanations. Many people say that the shudder is triggered when the soul of a deceased person passes through you or when an animal walks over the grave. 

Possible explanations for the shudder

Shuddering after urinating

The theory is that urine retention has an effect on the autonomic nervous system. The longer urine is held in, the more the body has to fight the urge to urinate and the more the autonomic nervous system is stressed. The nerves send signals to the urethral muscles to remain contracted. In order to urinate, the nerves have to tell the urethra to relax so that urine can flow. This reversal of nerve signals could be responsible for the shuddering after urinating. 

Another possibility would be the explanation that a large amount of body-warm fluid is released from the body during urination, so that the body produces new heat to maintain body temperature through the spasmodic contraction of the muscles.

Shiver before falling asleep

The sudden flinching in the transition phase between waking and sleeping is caused by the brain stem. Even if it usually appears that a fall or stumble in a dream has triggered the twitching, it is probably the other way round. The conscious and subconscious mind react to each other depending on the intensity of the day, so that occasionally the primitive part of the brain produces a reflexive shudder that has no meaning. In dreams, we give this shuddering a cause. The twitching occurs first and the brain finds an explanation for it, i.e. a sudden fall or stumble.

Arbitrary shuddering 

Some people experience these rapid and sudden muscle spasms at any time of day and in any environment. The body shakes briefly, then the "seizure" is over. It is possible that this muscle twitching is caused when the body stands or sits still for a long time without fully relaxing. Shivering is a way for the body to relax and give the muscles movement. This shivering is not the same as the sudden shivering caused by a cold draught.