Good old stretch

[Bazile]

Feels good when you have a good old stretch , doesn't it ?

But what is actually happening biologically , inside the body

[Clanad]

Our friends at The Mad Scientists suggest "...After awakening, stretching feels good because you are basically relieving the agonist/antagonist muscle pairs of their duties. Straightening out my arm will cause my
tricep to flex, taking up the job of contracting, while my bicep gets to relax. Many benefits come from stretching the muscles, the most immediate being the relaxation of muscles that have been constricted. The movement
of muscles (and limbs in general) promotes blood flow and aids in circulation (as you theorized correctly), including the removal of waste products that may have accumulated in the muscles (such as lactic acid).
Stretching increases tissue temperature, and also stimulates the synovial fluid in the joints, keeping them well oiled...."
Basically, all muscle groups consist of opposing muscles or groups of muscles. When you flex your arm, for example, one group of muscles works to move the arm while the opposing ones relax. When the body is inactive for a while (especially when sleeping) the relaxed muscles aren't able to dispose of waste materials as effectively as when being used. The stretching "wakes up" the muscle(s) and increases blood flow and removes the collected lactic acid (and other chemicals) as described above...

[Bazile]

Very informative Clanad

What actually triggers you to start stretching .
You can just be sitting for a while and all of a sudden you start stretching - is it just another signal by the brain ?

[Clanad]

The same site indicates "...An unconscious impulse arising from oxygen-hunger triggers stretching..." Interestingly, yawning is caused by the same factor and is considered (among many other things) to be a method of face stretching...