The logic of this myth seems remorseless and difficult to resist. It seems obvious that the more time you spend on something, the more work you will get done. From a numbers perspective this just seems to make sense. The problem is that human beings are living things and this variable significantly complicates the numbers. Research has shown that every hour someone works beyond the standard 37-40 hours in a week, delivers diminishing returns – you get substantially less out of hour 45, then you get out of 35 hours. In fact, as you get tired, the fact that you are more likely to make mistakes or miss important issues can mean your research can actually go backwards if you work long hours.
It is important to remember that being a PGR student is a marathon, not a sprint. Working extra hours for one week may well help you get a little bit more done but there will be a cost to pay in the following week. As you become more tired not only will you be more likely to make mistakes, problem solving will take longer as your thoughts slow and your productivity rates will drop. You will probably also find that although you are still at your desk or in the lab – you are spending lots of that time not doing any actual work.
Maintaining good productivity throughout your PGR requires you to maintain a good rate of energy – and for that you have to rest and take breaks from work.
Source
Summarised from 'The Wellbeing Thesis' - An online resource for postgraduate research students to support your wellbeing, learning and research - https://thewellbeingthesis.org.uk/