Working long hours can be important to get more done. But when you do it long-term, it can decrease your productivity. And in the long-term, you end up doing less than if you stuck with shorter hours.
For instance, in 1926, Henry Ford was the one who popularized the 40-hour workweek. His experiments had shown that decreasing the workday from 10 hours to 8 hours and the workweek from 6 days to 5 days increased total worker output.
You can do an experiment yourself to see if working long hours reduces your productivity. To do so, keep a time log to measure your productivity. With your time log, you can calculate the percentage of time you spend on non-work and work-related activities. (Read my article "Keep a Time Log to Discover How to Use Your Time More Efficiently" on how to keep a time log.)
Keep a time log every day for a week for different workweek lengths. For instance, you might do this for a 40-hour workweek, a 50-hour workweek, and a 60-hour workweek.
For each workweek, you'll have 10 percentages (assuming you work Monday to Friday). 5 percentages are for non-work activities and 5 percentages are for work activities. For each set of 5 percentages, calculate the average percentage (by adding them and dividing the total by 5).
With those averages, see if your productivity differs among the different workweeks. If you find that your productivity is lower when you work longer hours in the week (i.e. the percentage for non-work activities is high), try limiting your workweek hours.
Since 40-hour workweeks is proven to be the "sweet spot" by Henry Ford, does that mean you should stick to 40-hour workweeks? Not at all. It's better to find the workweek length of hours that works best for you. Try to find the longest number of hours you can work in a week while still maintaining a high productivity percentage.