The problem with saying "less hours" is that the optimal numbers of hours of effort varies a lot depending on your current productivity. If you can't manage to

The problem with saying "less hours" is that the optimal numbers of hours of effort varies a lot depending on your current productivity. If you can't manage to focus on something despite persistent effort, yes it's much better to stop and have fun instead. It makes no sense to have "long hours" as a goal even if you're not sure of what you are going to try to accomplish during that time. On the other hand, when you are motivated, or making progress on something, or working towards a deadline, then it may be very productive to keep going for many hours. I don't have statistics, but an important part of my PhD output (both research and writing) was done between 11 PM and 3 AM, on weekends, etc. (By the way, I disagree with the advice of going to the lab early in the morning -- personally I worked better from home late at night. But it depends on how you work and whether you can do the work from home or not.) Importantly, you should be in a flow mode, so actually these hours don't feel "long". The main productivity lesson I got from my PhD is this: either work productively, or relax wholeheartedly. Aka: "Do, or do not; there is no try." :) It's terrible to try to work on something and not succeed -- it's unproductive, unpleasant, it does not put you in a better mood like real fun does, and it makes you completely disgusted with the task so it will be even harder to work on it next time. You have to learn when to keep on working, and when to stop because you won't be productive working longer. (You get better at this productivity prediction with experience.) And you should be fine with stopping very early, maybe skipping the rest of the day if it turns out you're not in the mood; but you should also be fine with stopping late and working crazy hours when inspiration comes.