Pandiculation [stretching] is the involuntary stretching of the soft tissues, which occurs in most animal species and is associated with transitions between cyclic biological behaviors, especially the sleep-wake rhythm (Walusinski, 2006). Yawning is considered a special case of pandiculation that affects the musculature of the mouth, respiratory system and upper spine (Baenninger, 1997). When, as often happens, yawning occurs simultaneously with pandiculation in other body regions (Bertolini and Gessa, 1981; Lehmann, 1979; Urba-Holmgren et al., 1977) the combined behavior is referred to as the stretch-yawning syndrome (SYS)."

Source : Journal of bodywork and movement therapies 15(3):268-80

Involuntary stretching (of various body regions) very commonly occurs across a wide range of animals - mammals, reptiles and even fish. It's associated with tiredness, as it's frequently seen before and after sleepingplugin-autotooltip__plain plugin-autotooltip_bigSleep

"The purposes and mechanisms of sleep are only partially clear and are the subject of intense research"

Source Wikipedia"

Although extensive research in humans and animals has shown unequivocally that sleep is essential, the reasons why it's required are as yet unknown. Long term sleep deprivation not only severely impairs cognitive and motor skills, it's now been shown that it can cause physical damage to brain structure - and, in extreme cases, irreversible damage. episodes. The reasons for its deep-rooted prevalence (and its precise functions) are unknown.

It appears to be under some kind of reflex control, as (in humans) those who have been partially paralysed (stroke etc) are often observed to move their affected limb whilst yawning/stretching - but are are unable to do so under voluntary control.

As the paper cited above notes, stretching is deeply associated with Yawningplugin-autotooltip__plain plugin-autotooltip_bigYawning

The physical and psychological sequences which occur during the yawning process have been widely studied. Nevertheless, the reasons for yawning are disputed and unclear - as is identifying an evolutionary reason to explain the need for it.

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