What are pins and needles and why do they happen?

The medical term is paraesthesia, and the ordinary version is nerve signalling being disrupted and then recovering.

When you sit on a foot or sleep on an arm, sustained pressure does two things: it compresses the nerve itself, and it restricts blood supply to it. Nerves are metabolically demanding and depend on continuous oxygen and glucose to maintain the electrical gradients they use to fire. Deprived of that, they first stop conducting properly — which is the numbness phase, when the limb feels absent and heavy.

The tingling comes during recovery, not during the compression. Once pressure is released, blood flow returns and the nerve fibres begin firing again — but not in an orderly way. Recovering fibres discharge spontaneously and out of sequence, and different fibre types recover at different rates. Your brain receives a burst of chaotic input from a limb that has been silent, and interprets it as prickling, buzzing and sometimes a brief burning ache. It typically settles within a few minutes as normal conduction resumes.

Why it can be briefly unpleasant is that pain-carrying fibres recover on their own schedule, so the sensation can cross from tingling into genuine discomfort before normalising.

When it is not just positional: persistent, recurrent or unexplained paraesthesia can have many causes, including nerve compression conditions such as carpal tunnel syndrome, vitamin B12 deficiency, diabetes-related nerve damage, thyroid problems, and side effects of some medications.

Seek prompt medical advice for numbness or tingling that does not resolve, that follows a clear pattern in one limb or region, that comes with weakness, or that is accompanied by loss of bladder or bowel control. Seek emergency help for sudden numbness or weakness on one side of the body, facial drooping, or difficulty speaking — these can indicate a stroke.