Another question: Are you too tired to focus even a bit? I always try to practise at least 6 hours a day, even when I'm tired.
When I'm tired, I force myself to listen, instead of whining about how tired I am. That's the thing... If you find yourself whining, then you just need to focus more.
If you're focusing, but barely can stay awake, or that your head hurts for every tone you hit, then it's useless, cause that will damage the next day more than what it will help you the present day.
Also, ask yourself the question: Do I really have to? You'll find the answer pretty quickly.