I want efficiently use my daily time therefore want to decrease my night sleep to 6 hrs and to add may be 30 min nap in afternoon. what you advice?

Need 7-8 hours: Most people need 7-8 hours of sleep a day (although many are not getting that much). Within limits it does not seem to matter if it is all at night, or some of it is through a nap. You are aiming for 6 1/2 hours, which really is not enough. Not getting enough time in bed makes people tired and sleepy and reduces their cogntive ability.

Sleep more at night: For most people, 6 hrs per night is inadequate for optimal physical & emotional wellness, even with a 30 minute daytime nap. Enjoy a bit more sleep, and you will probably better enjoy your waking day, too.