As your breathing periodically stops during the night, the levels of oxygen in your body drop, signaling O2 receptors in the brain. The brain then tells the nervous system to constrict the blood vessels, which would decrease blood flow - and therefore oxygen flow - to the brain, heart, and other vital organs. 

The issue with this process is two fold. First, a consistent vasoconstriction throughout the night will cause a rise in blood pressure. Secondly, these nighttime problems have consequences that carry into the following day, keeping blood vessels tight to ensure that oxygen reaches organs, and therefore, a sustained high blood pressure throughout the day. This sustained hypertension can go on to cause atherosclerosis, which is a hardening of the arteries due to plaque buildup.

This serious issue can go on to cause issues as difficult as heart disease, heart attack, or stroke.

Another cause of the increased blood pressure could be from hypoxia - a drop in oxygen levels - causing a release of epinephrine (also known as adrenaline). If this stress hormone remains high, hypertension can result.

In a normal body, the blood pressure should fall during rest and then rise in the morning throughout the day, which is beneficial to your health. During sleep apnea, the patient has a consistently elevated blood pressure, meaning the blood pressure does not fall at night as it should. Besides the fact that an elevated blood pressure alone causes health problem, the absence of a decline in blood pressure at night means that the heart is not getting the proper benefits that one should get from a night of rest.

People with cardiovascular problems such as high blood pressure, heart failure, and stroke have a high prevalence of sleep apnea. Thankfully, the treatment of sleep apnea is directly linked to improvement of other, more serious, heart related ailments.

References:
https://health.clevelandclinic.org/2015/05/do-you-snore-how-sleep-apnea-can-hurt-your-heart/
https://sleepfoundation.org/ask-the-expert/sleep-apnea-and-heart-disease/page/0/1
https://www.health.harvard.edu/heart-health/how-sleep-apnea-affects-the-heart
https://myafibexperience.org/at-home/sleep-apnea

By: Jennifer LaBombard