implementing hook_node_postupdate

Hi

After enabling the module, adding the code implementation, clearing the registry, restarting apache/mysql and browser and cache.
I am trying to invoke the following hook, where submitting changes in node/'nid'/edit
My custom module is "my_module".

function my_module_node_postupdate($node) {
  watchdog('hook_post_action_test', 'The updated node @type id is @nid from @function.', array(
    '@nid' => $node->nid,
    '@type' => $node->type,
    '@function' => __FUNCTION__,
  ));
}

There's nothing on the watchdog.
Do you see something wrong?