A sleeping dog paddles its legs, whimpers and flicks its whiskers, then settles again. The movement comes from an identifiable sleep stage rather than from restlessness.
The stage where it happens
Mammalian sleep alternates between deep slow-wave sleep and a lighter stage in which the brain shows activity resembling wakefulness and the eyes move behind closed lids.
Dogs cycle through both, though their cycles are considerably shorter than a human's. A dog moves between stages several times within a period a person would spend in one.
Twitching belongs almost entirely to the active stage. A dog in deep sleep lies still, breathes slowly and is noticeably harder to wake.
Why the body is not fully still
During the active stage a structure in the brainstem suppresses signals travelling to the major muscles, which is what stops an animal from acting out what its brain is producing.
That block is effective but incomplete. Signals leak through to the extremities, which is why the movements appear in the paws, face and tail rather than in the trunk.
The leakage produces exactly the small, distal, uncoordinated twitches owners recognise. It is a sign the mechanism is working, not failing.
Age changes the amount
Puppies twitch far more than adults. Their proportion of active-stage sleep is much higher, and their motor suppression is not yet fully developed.
The amount falls through the first year and stabilises in adulthood. Very old dogs sometimes show an increase again as sleep becomes more fragmented.
Neither extreme is a concern by itself. What matters is whether the dog wakes normally, orients to the room and settles again.
Distinguishing a twitch from a seizure
The difference is observable and worth knowing before it is needed. Sleep twitching is irregular, confined to the extremities, and stops the moment the dog is roused.
Seizure activity is rhythmic rather than random, often involves the whole body stiffening, and does not stop when the dog is spoken to or touched.
Loss of bladder control, jaw chomping, and a period of disorientation afterwards point away from ordinary sleep movement. Any episode fitting that description should be described to a veterinarian, ideally with video.
Why waking a twitching dog is a poor idea
A dog roused abruptly from the active stage is disoriented for several seconds, and a startled dog in that state may snap before it has registered where it is.
The behaviour is reflexive rather than aggressive, but it accounts for a meaningful share of bites to children in the home.
Letting the episode run its course is both safer and unnecessary to interrupt, since the dog is not in distress and the movement resolves within seconds on its own.