Does lying down for such a long influence the change in my bp? if so , how can i regulate that?

There are certain types of low BP disorders that can be affected by laying down, or standing up quickly (orthostatic hypotension). Most High BP (Hypertension) situations are not negatively affected by laying down. But, spending too much time in a sedentary position can cause problems as the individual is not getting enough exercise which helps with high BP.

I think I answered your previous question. Spinal cord injury causes autonomic dysfunction and blood pressure increases when sitting up due to increased sympathetic activity. You will need to take Labetalol 30 minutes to 1 hour before sitting up to control the hypertension at erected position.