While doing Mantra Japa, is it normal to feel drowsy / sleepy /semi awake state and feel refreshed.
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 :)