If you're really tired now and you really can't keep going then rest. They say "never put off til tomorrow what you can do today" is true. You really must do it now, but what if you really can't go forward and you're so sleepy? What if you're almost dying and you can't think clearly anymore? Then it is time for you to say you're going to do it tomorrow but.... you can only do it if it is the first thing you will do when you woke up. The unfinished task should be your number one priority. Because if not... chances are you're not going to continue doing it. You will do something else that will make you postponed the unfinished task from yesterday again.
It is normal to get tired, it is normal to rest because you're just a human but if you want to finish your important task fast then you should do it first when you woke up, no more brushing of teeth, no more flossing or eating breakfast... do it right away because if not... your momentum will die, your brain will look for another activity to do and your task will be forgotten, you will no longer be motivated doing it.
This technique is effective if you have a project or goal that is pursuing. You can become successful fast if it is the last thing you are doing at night and the first thing you will do in the morning. It is like the law of continuity, you are connecting the past and the present. You are keeping the boll rolling, your mind is wired to finish the task or achieve the goal, you are keeping the consistency alive because you know that if you stop... then it is game over. Your ideas will die, your solutions will be forgotten, you will lose momentum.
Rest now if you must but stay lock in and never do something else tomorrow other than what you are doing today, stay focused and committed.