What Causes Dizziness While On Hypertension Medication?

My father is 86 years old. Just yesterday he suddenly felt dizzy early in the morning at about 2 am and behave very jittery and stress out. His blood pressure was taken at 7 am with a reading of 155/80. Currently on hypertension medicines. Is there any cause of alarm and needs medical attention? After resting for a few hours he was feeling very much better and did not seek medical attention.