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Counting sheep is a mental exercise used in some Anglophone cultures as a means of lulling oneself to sleep. It most likely arose from Yan Tan Tethera, a traditional numbering system used by some northern English shepherds to count their flocks.^{[citation needed]}

In most depictions of the activity, the practitioner envisions an endless series of identical white sheep jumping over a fence, while counting the number that do so. The idea, presumably, is to induce boredom while occupying the mind with something simple, repetitive, and rhythmic, all of which are known to help humans sleep.

Although the practice is largely a stereotype, and rarely used as a solution for insomnia, it has been so commonly referenced by cartoons, comic strips, and other mass media, that it has become deeply engrained into popular culture's notion of sleep. The term "counting sheep" has entered the English language as an idiomatic term for insomnia. Sheep themselves have become associated with sleep, or lack thereof. For example, an ad campaign of the Serta mattress company features the Serta Counting Sheep, a flock of animated sheep who resent the said company's mattresses for supposedly rendering their services unnecessary.

According to an experiment conducted by researchers at Oxford University, counting sheep is actually an inferior means of inducing sleep. Subjects who instead imagined "a beach or a waterfall" were forced to expend more mental energy, and fell asleep faster than those asked to simply count sheep. Sleep, by the same token, could be achieved by any number of complex activities that expend mental energy.

• Count Your Sheep, an online comic inspired by counting sheep
• Zzz is an onomatopoeia for sleeping in comics and animated television series as well as some live-action series; sometimes called catching some Zs
• Log sawing is also used in comic to represent sleeping