hi i have BSNL 500combo plan but when i use my net at night that is night unlimited time my speed decreases a lot than as compared to day time i.e downloading speed is only 5 or 6 kbps which is not supposed to be.

How you know that speed is only 5-6 kbps ?
Did you mean Kilo Bytes per second or kilobits per second ?
For me it is good, around 1.7 mbps in nights.

i mean kilo bytes per second downloading speed.

Do speed test as I suggested in other topic and let us know what it says.