The behavior you described of randomly putting your arm straight up in the air when laying down and letting it float there is a common and natural phenomenon. It's known as the "Palmar Grasp Reflex" or "Moro Reflex," and it is one of the primitive reflexes present in infants and some people might retain it into adulthood.
Primitive reflexes are involuntary movements that occur in response to certain stimuli, and they are typically present in infants as a part of their neurological development. The Palmar Grasp Reflex is one such reflex that is triggered when something touches the palm of your hand. When this reflex is activated, the person responds by tightly gripping the object or, in the case of laying down, by raising their arm with an open hand.
While these primitive reflexes usually disappear as the nervous system matures during infancy, some people may retain them to some extent into adulthood. The reasons for this retention are not entirely clear, but it is believed to be related to individual differences in neurological development.
It's essential to note that occasional presence of the Palmar Grasp Reflex in adulthood is generally not a cause for concern, especially if it doesn't interfere with daily activities or cause any discomfort. However, if you notice any other unusual or concerning behaviors or if you have specific concerns about this reflex, it's always a good idea to consult a healthcare professional for a proper evaluation. They can provide a more personalized assessment and address any questions or worries you may have.