Hello and thanks for the query.
The 1st Number: Systolic pressure is the blood pressure when the heart muscle contracts.
The 2nd Number: Diastolic pressure is the blood pressure when the heart is relaxed.Diastolic blood pressure changes frequently throughout the day. Things that can make diastolic blood pressure change within a few minutes include:
 
Posture
Level of exercise
Amount of tension
Nicotine use.
Therefore, it's best to use several blood pressure readings to arrive at your average blood pressure.
The diastolic blood pressure is considered low when the blood pressure reading is below 60.
The first condition that may result in extremely low diastolic blood pressure is severe hypotension (blood pressure less than 90/60 mm Hg). A number of medical conditions may cause severe hypotension. For example, heart problems such as extremely low heart rate (bradycardia), heart valve problems and heart failure can cause severely low blood pressure. An underactive thyroid (hypothyroidism) or overactive thyroid (hyperthyroidism) can also cause low blood pressure. In addition, endocrine disorders such as adrenal insufficiency (Addison's disease) and low blood sugar (hypoglycemia) can also trigger low blood pressure.

Some medications can also cause low blood pressure, including diuretics (water pills), alpha blockers, beta blockers, drugs for Parkinson's disease, certain types of antidepressants, and sildenafil (Viagra), particularly in combination with the heart medication nitroglycerine.

Other conditions, including dehydration, blood loss, significant infection and a severe allergic reaction may cause severe hypotension.

The other possible cause for extremely low diastolic blood pressure is very stiff arteries as in old age.
If you are not taking any medications and if you are not having symptoms,then your reading of 56 as diastolic blood pressure is normal and you should not worry about it.
I hope to have answered your query however you may revert to me for any further query.
Thanks and best of luck.

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