Under promise, over deliver to build trust. If you go through planning process like Scrum or some other BS, over estimate tasks so you factor in some extra time. Overtime, you will get to the sweet spot. As a PM, I totally encourage my eng team to do this. This keeps everyone sane. If a team member is working late nights and weekends out of necessity, then the company has failed to plan things properly. It all boils down to the culture at your company and team.
@jamesbond007 is correct. When I was a manager, I often told my employees that regularly needing to work weekends or over 40 hours per week was due to a failure of management (i.e. me). I also explained that sometimes situations come up that demand long hours, but if they happen very often - more than a few times a year - then that's just bad planning. It's a bit like someone getting surprised in their budget by a car repair one month, then a medical bill, then a tax payment, etc. If it's happening all the time then the person who made the budget screwed up and the budget needs some additional buffer to reflect the reality. Work is the same way. If a manager is constantly blindsided by unexpected issues that demand extra work, then they aren't planning well and don't have the staff to meet their obligations.
In my experience, there are a few different reasons this can happen. I'm sure this isn't all of them, but I think that identifying the reason will help you figure out how to address it.
A. If this is a regular issue that most people on your team or adjacent teams deal with all the time, then this is either due to poor management or a company culture that has extra time as an expectation. Is the manager who keeps asking for more bad at their job in other ways? Or are they just bad at saying "no" to the customer or higher-ups? Or is the team bad at planning and giving them bad advice? Or are you downstream of a process that's always late? Depending on the reason, you might take a different approach.
B. Legitimate issues can also lead to this. If you're in a highly dynamic field with low margins, then it may just be the nature of the business. I worked on a project where we took away business from the clear market leader, and were under huge pressure to show the customer that we really could do the job. They trusted our abilities just enough to give us the contract, but they also knew that building the product was going to push us. It took about 2 years to deliver a prototype that exceeded all of our performance requirements, and cost far less than the market leader. We had an innovative solution that the customer thought added risk, but we were confident and delivered. After that the pressure eased up.
C. High achievers / people pleasers tend to put themselves into this position. In many cases they know what they're doing and are willing to put in extra to get ahead. It doesn't sound like you're choosing to work overtime to get ahead, but are you putting the pressure on yourself even if it's not there from anyone else? For instance, if you enthusiastically sign up for more work while you know you can't get your current work finished - that's a problem you need to address yourself.
D. Unfortunately, I've also seen this happen to low performers as well, particularly in software development. Software is one domain in which low, average, and high performers *dramatically* differ in their output. I remember a couple of SW engineers who took a full 2-week sprint to do the work that an average developer could do in a couple of days and a superstar could finish in an afternoon. These people seemed to suffer from the Dunning-Kruger effect - they had no idea they were so bad at SW engineering. Side comment - I had a developer with over a decade of SW engineering expertise ask me a really, really simple question about they work they were doing. I asked if they had googled it. Their answer, and this is close to if not a direct quote, was "I'm not very good with google."
I don't know if any of those reasons might be the cause of your issues, but the point isn't to slot your issue into one of these boxes. The point I'm trying to make is that you've told us the problem description, but not the diagnosis of the source of the problem. Why are they asking for more than you can do in 40 hours? Answering that will help you figure out the right way to address it.
Good luck!
Oh - one other thing - I think it's also good to understand your place in the organization when you decide how to address this. Are they struggling - maybe leading up to layoffs - and you're a low performer? Or are they growing, can't hire enough good people, and you're a top performer? Having some idea of where you rank among your peers and how the business is doing are also important as you decide how to address this.