This piece is cross-posted on my blog here. I hear a lot of theories around how to work optimally. “You shouldn’t work more than eight hours a day.” “You can work 12 hours a day and be fine.” “It’s important to take weekends or evenings off work entirely.” “It’s best to immerse yourself in your work 24/7 if you want to be an expert.” Perhaps most well known is Cal Newport’s claim in Deep Work that “For a novice, somewhere around an hour a day of intense concentration seems to be a limit, while for experts this number can expand to as many as four hours—but rarely more.” Many of these theories are asserted with surprising confidence...especially since they contradict each other. At least some have to be wrong or more nuanced, and it matters which are right. I coach Effective Altruists who want to maximize the good they can do. So they want to know how much they can work before additional work is wasted (or just less valuable compared to extra time doing other things). They also want to know how much they can work before risking reducing their long-term productivity -- burning out from working too hard is a lose-lose for them and the world. So, I dug into these questions to see if I could find an answer. Short answer, there isn’t a lot of good research on the topic. Long answer, our best data comes from World War One factory workers (turns out you can do interesting research when your research subjects aren’t legally allowed to leave), but maybe we have enough information anyway to make educated guesses. At the least, we can run personal experiments. Here’s a summary of my findings and opinion, followed by the actual research. 1. Limits on total hours. First, as you work more hours, each hour becomes less productive. If I had to guess based on the research, I’d say there are steeply diminishing marginal value around 40-50 hours per week, and negative returns (meaning less total output for the day per additional hour) somewhere between 50 and 70 hours.