Unless your work hours are being explicitly questioned, you perhaps are being a self conscious to the point of being unhappy. There is also a difference between being physically present for 12 hours and being productive. I don't think anybody would for a minute guess you are productive for 12 hours. If you actually are, you are way more valuable to the company than they are paying you and they would be damn fools to fire you. (And you shouldn't even try to be that productive).
Is your work tracked in any fashion, such as Jira?