I joined Facebook when there were around a dozen engineers. There wasn’t any formal organizational structure back then but co-founder Dustin Moskovitz was unquestionably the head of engineering. He had built and deployed most of the critical technologies himself over the course of the prior two years. He was always around and happy to be involved in any technical discussion, which was great because very little of our deployment was standard by any stretch of the imagination.

It was so early that the company had only just started using source control; apparently a month or two before I joined they would simply edit files live on the Harvard server and shout out what file they were changing to avoid conflicts. As a consequence, the culture of engineering when I joined was prone to frequent interruptions. Thankfully we were small enough that it wasn’t a major problem. Most of us were involved in every major change anyways back then. Six months later (with nearly double the engineers) when I found myself challenged by some problem or another I turned and walked over to Dustin as I had countless times before. But before I could even say anything he loudly declared “not right now.” I was shocked. In fact, this was so unusual at the time that everyone around us was shocked. I walked slowly back to my desk.

This was a watershed moment for me and a turning point for our engineering team. Just as we had once gotten so big that we could no longer just shout out what file we were editing we could also no longer just interrupt one another to make progress. We now had to work hard to defend our productivity. That work was hard because it involved doing something that was a little socially awkward: it involved saying no.

Look back on your past week and assemble an honest accounting of your attention. Does the time you spent in each area mirror the importance of that area? This exercise is the true test of your ability to prioritize. It will tell you whether you are able to maintain focus on the important or whether you get distracted by the urgent.

This is not a theoretical exercise nor a pedantic one. Our ability to make the most of our time is the determining factor not only on how much we can accomplish but also how we feel about it. Have you ever been busy for an entire week and felt like you got nothing meaningful done? Either your expectations are off or your priorities are. You may care about certain things more than you had been willing to admit, or perhaps you are letting the squeaky wheel get the grease rather than having hard conversations.

It was hard for me to get shut down by Dustin like that, but it was the right thing for him to do and doing it the way he did created a culture where it was okay for other people to do the same thing. This isn’t a license to be rude; we still owe it to one another to make time for collaboration. As we work through the exercise of how we would like to focus our own time we also need to decide how we would like to work with others and be transparent about those decisions so it doesn’t create an undue burden for them.

Saying yes is so much easier than saying no, but nobody else will ever value your time as much as you do so you need to be a responsible steward of it. Say No to meetings. Say No to interruptions. Say No to things that are urgent but not important. Prioritize your time based on impact, be transparent about those priorities with those around you, and stick to them. And never be afraid to step back and take stock; time spent thinking about where you spend your time is an investment that pays for itself.