Hibernation slows the body so much that breathing and heart rate drop noticeably.

The longer version

A hibernating animal suppresses its thermostat and metabolic rate, so oxygen demand collapses, breathing slows to occasional shallow breaths and the heart may beat only a handful of times per minute. Body fat supplies the difference for weeks. A ground squirrel in torpor may look almost lifeless, cold to the touch and stiff.

Periods of torpor are interrupted by brief arousals whose purpose is still debated, perhaps for immune housekeeping or for repaying sleep debt. Rewarming is expensive, and those arousals may account for much of the winter energy budget. Body mass matters: very small animals lose heat too quickly to stay torpid for long, while larger ones can. The strategy lets small mammals survive winters when food is absent.

Where this entry sits

This collection gathers curious details about sleeping and dreaming, from nightly brain stages and body clocks to naps, yawns and unusual animal habits. It also covers how bedtime customs have changed across centuries, without offering any advice. The full list sits on the Sleep and Dreams page, where every entry is listed in order.