Thanks for being in touch. Regarding your question, taking tongkat ali on empty stomach is ok as long as you dont experience any stomach upsetting. It is a very concentrated product so for some people it is recommended after meals.
It is also recommended to be taken at night because tongkat ali can make you more active, hence keep you awake if you take at night. Then again, if you can sleep fine after taking it in the evening you can continue to do so.
Let me know if you have further questions.