I was in a similar position when I first moved to the UK. Landed a great job, but was literally doing the work of 3 people. For a number of reasons, I really wanted to make an impression in that job, so I started pullng 12-14hr days of pure focus and intense concentration. Within 6months I got promoted and given my own team to spread the work load. Sadly, I then got so bored with spending my days sitting in planning meetings and management pow wows that I left after a few short years, BUT, having shown commitment and dedication to the task and getting the job done, I was rewarded. Yes, your sanity is a priority, but your bosses are going to be a lot more flexible and accomodating when you tell them you need help, if you’ve shown them that you are, without possibility of reproach, making your task completion a priority.
I struggle with the concept of rigid 9-5 working hours. If I want flexibility from my managers, I need to show them that I am flexible for them/the company too. Particularly in commercially driven companies (which yours is), they want to know that you are as invested in their profit margins as they are.
Good luck, hope you find a happy medium!