For a very long time, beheading was used as a form of execution because it was believed it resulted in instantaneous death. For quite some time, there was suspicion that this wasn't the case, but many rules and regulations governing the use of cadavers limited doctors from thoroughly investigating enough to challenge the practice.
However, at the turn of the 20th Century, a French doctor, Beaurieux, was permitted to make an investigation of a severed head from a criminal named Languille, immediately after guillotining. He notes his observations:
*"Here is what I was able to note immediately after the decapitation: the eyelids and lips of the decapitated man worked in irregularly rhythmic contractions for about 4 or 6 seconds. I waited several seconds longer. The spasmodic movements ceased. The face relaxed, the lids half-closed in the eyeballs, leaving only the white of the conjunctiva visible, exactly as in the dying whom we have occasion to see every day [...] It was then that I called in a strong, sharp, voice: 'Languille!' I then saw the eyelids slowly lift up, without any spasmodic contraction -- I insist advisedly on this pecularity -- but with an even movement, quite distinct and normal, such as happens in everyday life, with people awakened or torn from their thoughts. Next, Languille's eyes very definitely fixed themselves on mine and the pupils focused themselves. I was not, then, dealing with a vague dull look, without any expression that can be observed any day in dying people to whom one speaks: I was dealing with undeniably living eyes which were looking at me."*
Every person who was ever decapitated was most likely aware of their predicament for a short time following their 'death'.