circulation

Sustained pressure squeezes a nerve and its blood supply, so the signals that carry sensation stop. When the pressure lifts, the recovering fibers fire on their own, and the brain reads that noise as pins and needles. The episode is common and usually harmless, but the same mechanisms underlie nerve compression syndromes, and the pattern of symptoms helps separate the two.

Your heart beats about 100,000 times a day without a single conscious thought. Here's the remarkable engineering behind it.