Well, yes. Yes it is.
But if by "in your head" you mean a perception manufactured by interactions of brain chemicals and synaptic patterns, then I ask you: what isn't? Isn't what we as biological beings perceive through our traditional five senses truly nothing more than our brain's interpretation of electrical impulses sent via our optic nerves, eardrums, taste buds, receptors in the nasal passage, and nerve endings in our epidermis?
Ever seen The Matrix? There's an elaborate, entertaining, and brilliantly told story based solely on the idea that the whole world we live in is a lie, fed to us via biochemical cocktails administered in the right order to produce a tailored perception. Ever taken a philosophy course? For thousands of years, men and women from every culture on our world have asked fundamental questions about what we are, what our purpose is, whether any of the things we think we are surrounded by our whole lives are even real...and what "real" would even mean if the answer was yes. Even heard of psychology? There's a science based 100% on learning how our minds interpret input and feed us a reality that may or may not be the same as that experienced by those around us, and then figuring out ways to manipulate the reality of those whose are different and make theirs more like ours.
So when you give me that look to test whether I'm joking, or ask me if I'm serious, or comment about metaphysical bunk, I will indulge you, and smile. And then you will return the favor by seriously considering that while you think you have your two feet firmly on the ground, and tell me about the things that are truly important and real, you are merely experiencing a similar illusion, custom made for you by your brain and fed to you through a filter of social expectations, cultural influences, and interactions with the people you've surrounded yourself with.
And then we can begin to truly communicate.