Thank you all very much.
I am going to pay attention to other parameters like diet, physical activity, degree of tiredness etc to see if they are causing the problem.
But one thing though. Sometimes, on the days when I do meditation, though I feel very tired, body exhausted, eyes feel heavy, I can't sleep. The whole system wants to sleep, but something just stays very alert. This does not happen if I don't meditate.
CarsonZi, I usually try meditation over the weekend because of the sleep problem. I get up very late the following day. So I don't know how I feel if I get up early. I will try one day.