Dont eat heavy meal before japam,if hungry have a light meal.

Also these things happen in initial stage for some,you have to put your 100% and go on.

You will surely be in a position to do it.

In "Gifts from the Goddess" when Guruji talked about the advice for meditation, which also applies in this case, he mentioned:

1. Be a Little Hungry

2. Be a Little Tired

3. Be a Witness

4. Be Expectation-Free

An hour of meditation is supposed to be equivalent to 8 hours of sleep! So if you are feeling refreshed surely you doing something right :)