1, Your brain might be tired. I used to sit and watch anime until say... 02:00 in the night, and i can tell you i was useless the other day.
2, Lack of concentration might cause it: Try to calm down, eat grapes, just get sacharoz or something in your system, drink a tea or coffee.
3, You might have other things in mind, forget them. Same for your problems, at least when you're learning.
4, You might be worrying that learning japanese takes away your time, and stops you from doing other stuff you should... well its true, but you didn't join the course for nothing.
All in all, when i can't study for some reason, i drink a tank as strong as hell, when it doesn't help i sleep, just set an alarm clock to like 30-60 minutes, that should be enough. When you did both and it doesn't help try to study while sitting, and not while in horizontal
that makes you fall asleep. Fresh air might help too, physical exercises...
Thats all i can think of, if none helps it means you're at your limits, just have a nice long sleep and continue the other day.
Oh yeah, have motivation, if you decide to pawn the folks at the course it surely will work out, even if you're near death you'll probably be able to concentrate when your pride is in danger
Tell me if none of this helps, i'll try to think of something else.